The 4star can make backpacks and a few other low level useful items, but nothing big.
The 3star makes XP, so sure it´s useful, if you don´t have the 5star mimics.
As heroes both suck and you sure don´t need them unless you are a newbie, in which case these 2 heroes are the best thing in the game for you, super cheap and quick to level up so you can start playing with better heroes than some random non-costumed S1 3star which is much worse and cost a lot more to max...