I like to charge my batteries to 4V for longevity purposes but i typically do it manually, pulling it off my current charger or if i forget using the light until the voltage comes down before storage.
Are there any chargers that have a 4V option or allow me to choose what voltage to terminate it at?
Low cost is the goal but its good to know what the higher cost options are as well.
The Skyrc Mc3000 can do that. You can select the termination voltage. I have one program set to terminate at 3.8V for storage… Or it will discharge to a certain voltage… again for storage 3.8V. It is not cheap, but it offers lots of flexibility. I have lots of other chargers, but I don’t remember any others that allow adjustment of the termination voltage. That feature would be nice in something at ~$50.
It looks good but its an expensive charger. Future stretch goal 
I’d be happy with a charger that has calibration ability that i can ramp down.
Special features needs special devices and special prices… !
It’s not 4V like you asked… but If you’ve a charger that does LiFePO4 cells, that setting can be used for a termination voltage of ~3.6V.
Edit: I remembered there was a post somewhere on here about someone changing the termination voltage one of those li-ion charging chips by changing a resistor… I can’t find it now, if anyone knows what the mod was, I’d be interested, I’ve some unused chips and 18650 holders, could be a nice project to make a standalone, cheap, 4v charger.
ISDT C4 Evo can be configured to terminate at 4.10V
Agreed. Cheapest I’ve seen it was $80 for a black Friday sale.
So that’s how I got mine lol
If it makes it any easier to take, mine is over 12 years old. I use it pretty much everyday.
If you amortize the ~$100 over 12 years (about $8.33 per year), it is not that bad . Do I sound like an appliance salesman ?? 
Yes 
Most no name Chinese brands will not give you this kind of longevity. I have been through multiple chargers.
That said i am a fan of paying for quality but you can’t know if you will get quality or money down the drain. Thus i assume any Chinese brand is sunk cost as soon as its spent.