AF lounge is indeed airside in T1.
I have no experience in that but I sincerely doubt you will be allowed through security at T1 with a BP for a flight leaving from T4. It is different from T2/T4 used by DL and which operate basically as a single DL terminal with an airside shuttle (they check BP at the shuttle but only the date), and you can move freely between the 2 (I did it a few months ago to avoid to go T2 SkyClub which is total crap).
This being said, even if it was possible, why would you do that ? OK AF lounge is nice and has just been renovated but what a hassle ! And you will need to leave T1 quite early. T1 is a pure international terminal and so I don't know if there is an easy way to go out (as you do in a terminal when you arrive from a domestic flight). And DL SkyClub lounge in T4 is one of the nicest in their network, so you should definitely go there directly IMO. And, again, I really believe that you won't be allowed in T1.