Vent details vary, but the four screws on the inside likely just hold the interior trim frame to the main body of the vent. The real installation to the roof uses many (my original had at least 16) screws, rivets, or (for those of us who prefer them) bolts, fastening the flange that sits on top of the roof to the fiberglass shell of the trailer.
The interior trim will likely need to come off first, then the vent comes off upward with the removal of whatever fasteners and sealant were used. Since a sealant is often used which is plastered right over the flange and the heads of the fasteners (mine was), the fastners may not be apparent now.
While vents are quite standard across RVs (even motorhomes), any specific unit could of course have something unusual - my comments are about the common design, which was in my Boler