Why an LCS cannot simply take a 4.5-inch gun

Could a modern littoral combat ship carry a 4.5-inch gun?

Not without major reconstruction because its deck and supporting structure were not designed for the weapon's weight and firing loads.

The mounting transfers static weight and dynamic recoil loads into the hull. A deck designed for a lighter weapon cannot safely absorb those forces simply because the larger gun fits geometrically. Making the change would require rebuilding a substantial part of the supporting structure.