Giving 5* weapons to merged 4* units

It's worth it if you have plenty of copies and never plan to promote them. For example, I pulled a 5* Soren and had about a dozen 4*s in the barracks. Merged away, now he's got solid stats to back up his weapon with no investment.

While I don't personally do this anymore (I only ever done this to my Olivia), the reasoning is that 4-Star +10 units are much cheaper than 5-Star +10 and have the same effectiveness as a 5-Star +5. (With a max of 40K feathers, you can get a super-merge unit with the highest level weapon that is better than a 5-Star +1 / +2.)

While your Arena team might prefer a fully merged 5-Star +10 than the 4-Star super-merge, these build are so much cheaper and can work in Arena Assault where scoring is based only on the first team.

If you really like Tharja and want to build her to the fullest potential, then you should just focus on promoting a bunch of Tharja and making the 5-Star super-merge. Otherwise, if you just need a quick and easy way to build powerful units for something outside of Arena (like Arena Assault), then give her Rauorblade+ to the 4-Star one and build that one up to +10.

Just like what everyone else says, Its worth it if you don't care about arena scoring and don't feel like waiting to accumulate the feathers. I want to eventually 5*+10 Effie and Cherche

If you're thinking about making Tharja one of your main units and want to invest a lot into her, then save the 5* version and promote the others to 5* for a supermerged OP Tharja.

If you're looking to just build some strong units while saving some feathers, then yes I'd merge the 5* into the 4*+10 for Rauorblade+. Personally I have a ton of 4*+10 units partly because I just like building units but also because I like having a variety of units with at least one specific niche (ie. Green armor counter, blue blade tome, etc.) for arena assault. Finally finished my latest project, Merric :D