I have a 4* +Atk -Def Tharja... should I 5* her?

Depends on the units you have in your barracks. For a non-whale, it's typically best to try for 28 or so 5*/4*+5 units with at least 7 of each color for the sake of Arena Assault.

If you have 7 for all 3 non-colorless colors, then go ahead. If you haven't reached your 7 quota for red units yet, go ahead.

That said, if you just like Tharja, upgrade her anyway, what's the harm? She's a strong unit and there's nothing wrong with waifu favoritism. Optimal play, upgrade her if you have enough blue and green. Casual play? Do what you want! Knock yourself out.