The F4 is built like a tank. The F100 has the more up to date features and better advancements in a lightweight body, but wont be as durable as an F4 which you could chuck off a mountain and it would still work fine, far fewer electronic components to fail aswell. I recently had this choice to make, and chose the F5 over both of these due to the solid body, super fast af and excellent durability and great features. The F100 can probably do the same in the right hands, but it wont match the F5 for AF speed or body.