A 4* level 40 + 10 unit better than a 5* level 40 ?

Yes, you can.

Yes, a 5* naturally has 2 more of every stat vs a 4*
Making a 4* +10 would add +4 to every stat, making the 4* have 2 more all around stats than the 5*
But also consider that being 4* limits the skills they can learn

https://fireemblem.gamepress.gg/creating-4-star-merged-unit

A +5 4-star is about equivalent to a 5-star in strength.
A +10 4-star is about equivalent to a +5 5-star in strength.

For more info check it out.