Sly 2, 3, and 4 (going by impressions and demo) are all roughly the same thing. The apathy towards Sly 4 is just that you're going through the same motions again; same highs and lows. Looks like a well made game and worlds better than the non-ND/Insomniac Jak/Ratchet games, sans Daxter. Depends on how much of the same game you're willing to play. Some people hit a limit. I hit my God of War limit with Castlevania LoS. Doubt I'll ever play that second PSP game or judgement.
anyway, I'm currently going through Sly 1 again, HD version, and it's seriously great. Went through Sly 2 HD last year and in some ways it betrayed my memories because you tend to block out the padding and annoyances (The Wind Waker Effect). Sly 1, if anything, is better than I remembered it. It's really good.