4*+10s for Arena?

They DO score in Arena.

But they "don't" score cause they score less than 5+10 so maybe that what that person meant.

Everything counts for Arena scoring. 4*+10s just count less than 5*. While 4*+10 has the same stats as a 5*+5, the arena counts is less because it's still a 4*. The change is very minor though, it's equal to the difference between two base stat total bins (165-169 & 170-174) or half a merge.

So if you've got a character for the arena and they're currently 4*+10, but you have enough feathers to +4 them, it isn't worth it (To promote) because you'll lose score. Wait until you can get them higher.

You can see exactly how it plays out, with and without teammates, using a calculator. I actually haven't tried it myself so I'll let you have at it.
http://www.arcticsilverfox.com/score_calc/

Look at it this way. Can you really expect 10x2kfeathers to carry the same weight as 10x20k feathers? That's 10 times the investment cost, so you should expect to see the same result in arena scoring.

The score-weight that a 4* carries is insubstantial compared to its 5* counterpart, and rightly so.

A 4*+10 will have stats similar to a 5*+5 and carry arena weight somewhere between a 5*+4 and 5*+5, because of lower base unit BST and because it's a 4*. Such units are probably sufficient for your arena core up to around tier 19 or 19.5 (get to 20 but not stay in it). They're also fine for bonus units.

For units you'll eventually want to merge at 5*, you should think twice about merging them at 4* in case you can't get enough of them later.