AOS made it to four seasons for reasons that have nothing to do with critical reception. First, it had one powerful backer, Disney CEO Bob Iger. He proposed the series in the first place because he wanted an MCU tie-in on ABC. Second, four seasons will give the studio enough episodes to package the series for cable syndication. That will bring in a lot of money once AOS is sold to TNT or wherever. Plenty of other low-rated shows have limped over the four year mark for precisely that reason.
BTW, that 100% RT rating looks impressive but it represents only 13 bloggers/publications. Many of those outlets are obscure or genre sites. And some of those positive reviews are mixed or are only for single episodes. Put in that perspective, AOS isn't the critical darling that the perfect score implies.