Each merge will raise two of the hero's stats by one point each. If you reach +10, the end result will always be +4 in every skill. As for the order in which the skills grow, I believe it depends on the fodders you use. At least, I have confirmed the +1 merge of certain units does not always yield the same results.
Oh, and if you want to 4* +10 the unit, promote one copy to 5* before merging it into your main copy and you will have access to the 5* weapon too.
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Fate Grand Order
Dragonball Legends
IV Calc
Heroes