It is normally a problem installing Seitz S4 windows into panel vans as the ribs inside the vehicle, mean that your wall thickness exceeds the recommended thickness for these windows. However there are not allot of alternatives on the market so you will find just about everyone fits a S4.
You will see allot of people simply discard the screws provided and use longer screws to make up the gap, then trim the gap between the inner and out frame. The only problem this can cause is that the outer frame had a nice smooth surface of the blind and flyscreen to roll against this stops the possibility of them becoming un tensioned. It also goes against the manufacturers instructions and if there is ever a problem with a blind or flyscreen it may invalidate your warrnaty.
How we do ours is to make a frame from standard 25mm timber, in this case we are installing a 900 x 500 Opener, so the frame wants to be 900mm x 500mm internal.
We then build a sub frame from the 900 x 500mm timber frame, so our new sub frame finishes level with the internal ribs. What this allows us to do is recess the window into our walls and makes the frame surrounding this window flush with our finished internal wallboard.
We then bond the frames to the inner of the vehicle, (this is not really a necessity as the window will hold it in place, however as we have the luxury of time so we can bond overnight, meaning we do not need to hold it in place when installing the window)
As part of the frame and sub frame will be on show we have then lined it will some of the same wallboard which we will be using to line the whole vehicle, we do this before installing the window so it will fit nicely behind the frame rather than us cutting round the frame
All that is left to do now at this stage is again using Black Sikaflex 221 put a bead of the sealant round the outside window frame, get someone to hold the outside frame into position. Then from the inside locate the inner frame and screw together using the screws provided.
As easy as this screwing up of a window sounds there is still a nack to getting it just right, if you over tighten the screws then the blinds and flyscreen will not slide freely, so check after each screw that they are not causing any problems
You should also have some excess sealant squeezing out between the outer frame and the van, simply use a rag and white spirit to clean this up and point up the sealant