4 star Lv 40+10?

4 +10 is similar stat-wise to a 5 +5, so a 5 +10 has 10 extra more stats than a 4 +10, and the most significant difference is that a 5 +10 hero will have a fancy frame around their portrait. The difference in arena scoring is so obvious that I probably don't even need to point that out. The feather investment is mostly worth it, if you like the character that you are going for.

Well the stats of a +10 4* has definitely lower stats than a +10 5*. People really don't want to spend 220k feathers on 5* Merge Project as they simply think is not worth it. Ive almost finished a +10 4* merge project as well.

A 4*+10 has the same stats as a 5*+5 merge, which is +2 to all stats of a 5* I believe. A fully merged 5*+10 has a +4 increase to all stats so you get the full potential of the unit so yes, it really is worth the investment of 220k feathers. It looks like a staggering amount at first, I was sure I never would get that amount when I started playing over a year ago, but once you start you'll see that this game has tons of ways of earning those feathers and you'll gather them in a few months if you play regularly. Its worth the wait of collecting feathers, merges, and skills to build the unit you want and totally rewarding when you finish. Think of it as like a super-saiyan-steroid-venom-pumped version of your favorite character

It really depends on who the character is. Sometimes there is 2-3 stat difference. Sometimes there's 4 but it's rare.