I use the P26A in my both my D4’s, use to use it in FW3A as well. Got rid of the Al ones. Just order a copper one. Worked very well in both. Run times suffer of course. I am not always looking for max achievable brightness and find myself using LG MJ1’s in most lights. Lots of run time at 1000lumens or less. They are usually a bit pricey but they come up on sale sometimes. Lots of fakes on Ebay, Ali, etc. So I make sure I get em from IMR, Illumn, Liion, or Battery Junction.

I think IMR has the LG HG2’s on sale right now. $3.99 a piece. 2 battery minimum. https://www.imrbatteries.com/lg-hg2-18650-3000mah-20a-battery/