I’ve been chewing over this 4* vs 5* issue a lot. I agree with @Pois1 that those of us who have lots of maxed 5* are not going to use emblems on 4* — with some possible exceptions for 4* with extraordinary specials (e.g. maybe Wu Kong makes the cut, if you don’t have Tarlak or Santa).
For mid-tier players, though, the math is pretty straightforward: use ~1500 emblems to create three 4+19 or one 5+19. (Step 20 is stupid expensive and for most classes should be skipped.) A 4+19 is pretty close to a cuurent maxed 5*, which is to say really good. Adding three “quasi-5*” to your roster probably does more than adding one quasi-6*.