What trap are you referring too? If the base is not strong both will deform the frame itself. Going up one size usually distorts the levers. A quality trap made with good steel pays for itself over and over..

I would not put larger springs on any trap. They were designed and tested with stock springs. If the springs are weak, you can just replace them with brand new springs that are the same size and you will have a much stronger trap.

As far as 4 coiling traps, that is fine, but use springs designed to be added either by the manufacturer or another company. I would only four coil a couple traps and see how they work first. If they deform the frame or base then they cannot be 4 coiled without adding a base plate. Now the expenses add up.

As others have mentioned, just get traps that come like you want them. They will be designed to work that way. You can spend a lot of time and effort working on traps and have them not perform. You can always sell your traps in the trap shed if you want to upgrade.

A well designed 4 coil is better than putting oversized springs on a 2 coil trap in my opinion.

Why is that?

I think frame bending is more from setting traps on knees than springs. Think about where the pressure is being applied on the trap, it's on the cross frame by the pan posts and on the jaw levers. How does that exert pressure on the frame itself?