I have a laptop with similar specs which runs most other games fine at low-medium settings at a laptop resolution of 1366x768, AC4 is an exception which runs at around 16-17FPS (20FPS or so at 800x600) on the lowest settings so I'd blame the game for being poorly optimized
You don't need a mid-high end PC to at least play some games at lower settings at a laptop resolution, and given that Crysis 3 runs better than AC4 on low-medium settings with the GT 635m, it's the game itself and not the hardware that I'd blame
Laptops are definitely inferior for gaming given all of their restrictions, but I'd at least expect it to run them at the lowest settings with a playable framerate of 30-40FPS if you have a middle end dedicated laptop GPU