Merging 4*+10 into 4* will it become 4*+10?

I'm pretty positive it does work. Like 99%. The only reason I have a bit of a doubt is I never actually did it myself but logically it should be like that and I'm pretty sure I red it somewhere, yup found it!

https://feheroes.gamepedia.com/Merge_Allies

Thinking about it, you should be able to preview the result of a merge! Try it

Thinking about it, you should be able to preview the result of a merge! Try it

it will work! you can try it with random 3*s if you want to test it out

Fun fact, not only does it works, but if you merge a merged higher rarity unit into an lower rarity unit, it will add to the merge a the lower rarity unit. Like if you merge a 5*+2 unit into a 4*. it will give you a 4*+3 unit. Not usefull, but funny.