Hello again from your other posting you seem to ask the questions I have some experience with , I cannot say for certainty that any T4's had height adjustable function if they did you would be best check a Caravelle as it is the top specified model .You may need to replace the whole seat and its mounting base as they will be different to a standard one it will bolt to the original holes though . As for fitting another seat whilst possible you should get an engineering certificate for legal and safety reasons , if registered in NSW it is pretty cut and dried that you will need to use an approved fitter to certify the job with an engineers certificate $$$$. If Qld registered you only need an authorised officers certificate for an alternate seat but it must be fitted to same standards as factory and of equivalent age or later . You can do the fitting yourself and make sure its a good job and then get it certified . How much height difference are you hoping for , as I used to be a motor Trimmer {retired don't open up that wound } we used to lower seats by cutting away some of the seat foam in the base naturally there is only so much that can be removed before reducing comfort too much , but it did work as well . As a worst case scenario a couple of sports seat Recaro/ Corbeau /Sparco etc may be another option as most of these have an open base design which would work well in a Transporter .