I use reksai. After i conquer with unit that has assault 4, and then i reveal top 2 and look at death grip. How many might does i get from the unit that just conquered?

It's the unit's full Might — base plus 4 from Assault. Deathgrip reads "+[M] equal to its Might," and Might is evaluated at its current value (Rule 710), not the printed one.

The reason Assault still counts: conquer triggers are placed on the chain at rule 468.6, and Attacker/Defender designations aren't removed until 468.7 ("Combat ends"). Rek'Sai's reveal-and-play and your Deathgrip all resolve inside that window, so the conquering unit is still an Attacker, Assault 4 is still live, and you transfer base + 4 Might to the other friendly unit (plus draw 1).

One caveat: this only holds if the conquer came out of a Combat Showdown. If the unit conquered an empty battlefield in a Non-Combat Showdown, there's no Attacker designation at all, Assault never applies, and you'd only get its base Might. See FAQ #11660 for the same timing with Cleave.