If you get 120% or more on 4/5, keep it on that size since is 99% of the next fruit will lower the size.
About the purple aura, you get it by size % and in 120% case, minimal cm size. Now about 4/5 or 5/5, the only diference is that only 5/5 can get the max size of 130%, and can surpass 120%+ easily (normally they hatch already 120%+). But if you, for example, pick a 5/5 dober with same % or yours, will have same base stats.
BigTortoise (GDMO Leviamon)
Guild: Outragers - GuildMaster
39 Mercs - newest: BlackHayate, BkGabu 4/5 117% (Shiro, the Gabu got a brother xD)
Biggest: Kuwabara, Bear 5/5 126% (I think I'll stop fruit him)
Level Ranking: Kuwabara, Bear 5/5 126% Top 6; Odin, Goma 4/5 118% Top 8
First hatch: Undertaker, Drimo 4/5 118% (event); KuwagataX, Kune 4/5 119% (merc)
Luckiest: Destroyah, Kame 4/5 119% (2 normal core eggs)