Following on from the post for the Mk3 (please read first) I have done something similar on our Mk4.
The seat cover has the same zip arrangement as the Mk3 but when you get in there it is a complete void, the seat cloth is all that is holding the tension against your body. Fortunately the metal construction of the seat back frame has conveniently included ‘slots’ down each side. I cut a plastic number plate to just under the width of the back frame between the slots and by bending the plastic number plate I could insert it into the slots so creating a false back to the frame. I fed one side in first to limit the amount of flex required to locate the other end.
Once inserted you can insert padding of your own choosing between the seat covering and the plastic number plate. I have used one of these https://www.amazon.co.uk/Original-McKenzie-Lumbar-Roll-Packaging/dp/B003WGIE9S/ref=pd_sim_364_5?_encoding=UTF8&pd_rd_i=B003WGIE9S&pd_rd_r=71ccab63-76ef-11e8-9402-d3b71531a493&pd_rd_w=7yFtG&pd_rd_wg=mzqWk&pf_rd_i=desktop-dp-sims&pf_rd_m=A3P5ROKL5A1OLE&pf_rd_p=3274180622111699416&pf_rd_r=V787Y1FJA1XKK0PERF7V&pf_rd_s=desktop-dp-sims&pf_rd_t=40701&psc=1&refRID=V787Y1FJA1XKK0PERF7V. and some memory foam pillows.
I used two cut down number plates on each seat. Obviously the filling you use is up to you and if you need less ‘give’ than is afforded by the number plates you could experiment with less compliant materials.
Take care not to trap any wires or other fittings within the seat frame when inserting any materials into the void.
Having used what we had available the results are significant.
I hope this helps those who find the lower back support somewhat lacking in the Mk4.