The difference between a 4* and 5* will be about 10-12 points spread across all of their stats, excluding the MT of their equipped weapons. If you have a 4* that you see yourself promoting to 5* in the future, you may want to level them up for the extra SP.
Pokemon GO
Fire Emblem Heroes
Fate Grand Order
Dragonball Legends
Dragalia Lost
IV Calc
Heroes