Hi Laggan, If it is only the dash top speakers you wish to replace you should be able to get some for £30 or less .
I got some Pioneer ones from a local audio centre near me for £30 fitted, what a difference it made, still yet to fit some 6 x 9s.
Admit if you wish to source door pockets for your T4 this will cost in the region of £80 for both i would think, Add to that price for speakers, wire them up etc.
Depends how much you wish to spend, remember that you can only fit passenger side door pocket if you have a single seat though ?
Regards, Steve W....