What to use as feeders?

Should 4*s ever be used as feeders, or is 3*s the highest you should go?

What's the best way to max out a unit?

Thanks in advance

4*s are usually feeders for 6* units, otherwise you would just use 3*s

did you ever get an answer? I just got my first 5* and I want to max it out as quickly as possible. I was wondering how many lvl 38 4*s would max out a 5*. or how many lvl 38 3*s would max it out

You would need 19 level 38 Feeders to max a 5*

28 lvl 38 3* to max a 5*

Source I used: https://docs.google.com/spreadsheet/ccc?key=0AoArAI8XBLehdHRiMWJUNHpyYzdqRkFNa2pSdDFPWHc#gid=1