The obvious answer is to train your Robot Masters to level 100. But if you really want to push their stats beyond their limits, you'll want to go star-hunting. Each elemental star you collect will increase the stat limits of
a robot of that type. Isee you're already way on your way to collecting them and as a result, your robots have steadily been growing stronger. It's a long road to collecting every star and you quite frankly don't need to collect them all (nothing in the main campaign will ever really require that you do), but regardless if you want your robots to get really strong that's the way to go.

Does he know about the 4 Star Robots? (Is there a 5 Star)

i already have 100lv team robots and types for any cases with the 4 stars
i don´t have the 5 star but i am upgrading a lot the starforce thing

Oh btw the grinding is really heavy (1024 stars rn) so you need a lot of dedication
It is really satisfying though