Like your friend I’d never go below 100ft. Those cells look good if they can do 8A continuous, and of course a protection circuit is a must.
Also being a dive light it’ll have to be able to run for 2 hour continuous without frying the internals, hopefully the cold water of Sydney should prevent that from happening.
I have one and have dove with it many times. I use it attached to my spear gun . Never had any problems, I find it bright and reliable. I like the magnetic switch and have never run out of batteries using 2x26650 protected 4200mah 4sevens batteries. Make sure you grease up the o rings on the tail and tighten up the head before using.
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I also have the D35 that i have never actually used (for anything including diving), its way to big and heavy to use on the spear gun and I almost always dive with a gun. The DX4 is a much more functional design IMHO. One day if i do a night dive in a spot that i cant hunt i might use it. Actually if you want it, let me know I would be willing to sell it to fund my BLF Titanium build 
Good to hear! How deep have you gone with it?
WRT diving, would spotty or floody be better? I was thinking dedoming the LEDs might make it awesome!
I added a tail spring 22 AWG bypass, tightened up the bezel, applied Nyogel to the threads/O-rings, and re-tested with fully charge KP 4000's (@4.19v). Got better #'s.
Still 4.8A at tail, lumens: 3,791 @start, 3,675 @30 secs. Nice bump!
Did a 15 minute hi run in a bucket of water - wow, that was bright, even in a bucket! Everything survived well, light stayed cool, did a few mode changes in the bucket. Wiped it down, then took more measurements:
Cells at 3.86v, tail: 3.6A, lumens: 3,427 @start, 3,325 @30 secs. Overall not bad at all. I believe this is ready to go to the next level of testing 
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Some pics of the driver, modded:
Close-ups of the R120 sitting on a R160, with the other R160 next to it:
Nothing done here, just fyi:
I never go much below 100 ft so its never been below that (for very long ;). It depends what you want it for. I like throw for hunting but massive flood for photography and go pro. This light is a good balance between the two. I put a white silicone cap on my lights if I’m going to be taking pictures or videos only, i was thinking about making a mule with no reflector for a dedicated video light but have not got around to that yet.
I wonder what the total runtime would be using 2 big capacity 26650’s…?
Has to be at least an hour
I don’t know, I have done 2 dives with it and forgot to turn it off for a while between so at least 2hrs for sure, thats all you really need from any dive light IMHO.
Actually if we take Tom E ’s measurements of the stock light at 2.4A on high, times 2 hrs = 5000 mah so yes a good capacity 26650 should just about do it.
Yes, I meant for the modded version. 2 hours is the minimum for burn time for me, sounds like 2 5200 mah 26650’s should do it…
I use KeepPower IMR5200mAh cells in mine and on high, the run-time is 3 hours. When the cells are nearly depleted, it starts blinking over and over again, even after going to the lower mode.
I have yet to use mine for diving purposes, but I'll only get down to 25 meters, or possibly 30 meters if the DM allows it. :)
The blinking sounds annoying for sure... I'll do a longer test on the 4000 mAh cells I have to hit the LVP level. Best cells for the mod light will be those 5200's, either KP or EVVA, protected of course. Hoping the 5200's won't hit as high current draw, so will lose some output but gain runtime. This light's driver is nothing too fancy - if it shows advantages for the spring bypass's, then it's not regulating very well. Of course might have to do with my resistor mod 
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Ohhh - sorry, think I was getting the models confused. There are two models:
Dx4: older model sold at CNQ and dis-continued at FastTech, head diam: 62mm (from FT), mag ring for modes
Dx4S: newer model, sold at BangGood, FastTech, KD, GearBest, head diam: 55mm (measured). thumb slider for modes
I have the Dx4S, bought from GearBest, that I've been modding. If I buy more, I'd buy from BangGood (banggood.com/Solarstorm-DX4S) with the 8% BLF code.
I did a total of 75 minutes in a bucket of water on hi with one pair of KP 4000's, and still going... Though amps starts at 4.8, it drops while the cells drain (driver not regulating well), so it's expected you will get over an hour out of 4000 mAh cells, but at well reduced output near the end.
After 75 mins: cells at 3.3v, tail amps: 1.65A
Update:
Another 13 minutes, the light finally died. Apparently a LVP triggered in a battery's protection circuit because it measure 0 volts. The other cell was at 3.1v. So total runtime with the mods:
1 hour and 28 minutes (8 minutes total)
In the last couple of minutes, I did notice a strobe effect, like a flickering. Not sure how long it was doing that.
Awesome results man! Brighter and a quarter of the price of an Xtar D35!
Like I said, would love to buy a modded one off you should you feel like modding any more of them!
Would you expect 2 KP 26650’s 5200mah to be able to provide enough amps?
Runtimes should be pretty close to 2 hours with the extra capacity
Yes - I'd like to mod one up for you. I can get expedited shipping from BangGood on qty 3 with the 8% discount, so qty 3, with one being for you, would be perfect for what I need right now. I'll place the order today. For the LED's, I can swap in XM-L2 neutrals, planning on leaving the stock MCPCB in - seems to be working very well, and I'm hesitant of messing with the vertical clearances if I change the MCPCB's to Noctigons or SinkPAD's.
I'm guessing the 5200's would be less top amps, but would not drop out as fast as the 4000's. The 4000 KP's and 4000 EVVA's are basically King Kong cells, so KK's are pretty low resistance and combined with a quality protection circuit, they have excellent performance for low resistance, high amps. Yes - I'm guessing the runtime would extend 25% on capacity alone, maybe longer, all depending on the discharge curve on how amps drop over time on these cells.
What would you suggest for Neutral tints? U2-3C, T6 4C, T4/T5 5B1's? The 4C and 5B1 are pretty cheap from Simon. I can probably get the U2-3C's bare from Hank, or quicker pre-mounted from Richard.
Also - can probably get the T5 5D3's bare from Hank, in the $5 range.
Hi Tom, are the 4 LED’s in series?
Looks 2S/2P to me. Didn't do measurements to confirm, but from traces, looks that way.
So that would be about 2.4A to each LED, is that correct?