Shoretrooper 3* vs 4*

I just got Shoretrooper at 3* and I have heard he is viable at 4*. Honestly I dont understand how a 3* is not viable if 4* is. Seems like gear and mods make a much bigger impact than 1 star level. Can anyone tell me if they have a 3* that is working for them or if I really need to bite the bullet and take him to 4*.

completely f2p so 4* is as high as I am willing to take him.