@ Cheese123
Let's see your 3 texture files. You are probably right, the Occlusion map maybe at fault. How did you generate it? 3ds Max? or similar?
The first thing that stands out (and this will be unique to all imported vessels & skins) is that the camo scheme, which is applied to the Normal/Colour map in SH3 is NOT implemented in the same way in SH4. Camo & Colouring must be applied to the Occlusion (light) map, otherwise, as with your screengrab, you'll see that the hull stripe has been bumped. Alternatively, is you think about it before, generating the Bump map from the Normal before a camo/colour scheme is added might also work.
To correctly import SH3 models into SH4 (to an SH4 standard) requires knowledge of either Photoshop texture creation or Max/Maya Bump/Occlusion rendering (to texture) from a lit model. However, in this instance, the only way to create an Occlusion map is to render one from a 3D Suite. Once you nail the process down it's quite easy. For additional added effect, render a Light & Shadow map and then merge them, the final result will be better than each individually.
