Is it best to unlock a hero's potential to 5* and merge lower rarity heroes into it, or merge them into a 4* hero?

tl;dr: If you have 6 of the same 4* unit, then the resulting +5 merged unit will be roughly equivalent to a vanilla 5* unit. If you have more than that, then a super 4* unit will have higher stats than its 5* rarity without merge bonuses.

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In general, it really depends on the unit and its general utility. For instance, some heroes do not natively come as 5* summons but are immensely powerful in the Arena, such as Cecilia. The cost to get a 5* +10 Cecilia, assuming you get 11 (1 base plus the 10 merges) 4* Cecilias would be 220,000 feathers. However, due to the fact that she is a relatively common 3 and 4* summon, you can make her more powerful than a base 5* Cecilia by merging a 4*+5 Cecilia or greater. The general consensus is that unless you plan to merge past +5 as a 5*, it is more beneficial to just merge a super 4* unit. In this instance, you'd spend 0 feathers making a 4* Cecilia that is roughly equivalent to a 5* +5 Cecilia that would've cost 120,000 feathers.

Some other heroes that can be merged into 4* units usefully are listed in this guide. These heroes include Olivia, Nino, and Donnel.

A succinct pros and cons list can also be found at that link:

Benefits

• 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.
• Lower feather cost.
• Easier for stat hunting.
• Rated highly in Arena scoring after fully merging, higher than a vanilla +0 5-star.

Limitations

• Once a character is +10, it cannot get stronger, as upgrading that character will lose your entire merge bonuses.
• A 5-star will be stronger if there is enough merges (+6 or higher).
• Most of the strongest characters are 5-star exclusives.
• Lack of access to legendary weapons unless you merge a 5-star version of the same character (for example, Chrom and Eliwood).