ps5 will be backward compatible, so you buy it now and play it again on ps5 without paying anything more.So the real debate is whether to get it on the PS4 Pro when it releases...or wait for the PS5 collected edition...Decisions Decisions.
P.S. TLOU remaster is also given with ps plus in october, which is what I've been waiting since TLOU2 announcement. I've already played it on ps4, refused to pay it twice, and now I'll play it again with the DLC (which I haven't played)