Oddly, the user manual doesn't appear to clearly define that. I'd guess they will be in the order they are on the tech specs, so x4 on the upper and x1 on the lower of the pair. Shine a torch into them an look for the gold contact fingers; the x4 will have more contacts than the x1 (assuming they have done the normal thing where it's only the plastic of the slot that is x16, and the contacts are not actually present for the missing lanes).
Ok, my guess above was wrong. I found the answer in one of the product photos, where the slot contacts are visible. The upper one is x1, and the lower is x4. I've highlighted the contacts in the picture below with a yellow tint, so you can see what I meant about looking for them.