On a 2wd cossie the small tube runs to the water cooling feed pipe for the turbo and joins a metal pipe that runs from the side halfway up the turbo to underneath and then heads towards the thermo housing where it joins the little piece of pipe on the thermo stub, I am not even 100% sur if the T4 is w/cooled ?, there on on the early thermostat housings TWO STUBS on the side of it both pointing towards the turbo, one is for bleeding the cooling system of air upon refill etc a tube is put on there and held upwards but not really needed, drill a small 3mm hooes in top of thermostat instead!!, later ones had only one stub.
I don't know 100% where the connection is on the T4, but so long as it goes on the HIGH pressure side, ie the side with the boost pressure, that will be fine as it is this that it needs to know, so that is anywhere between turbo compressor and plenum, so a stub on the plenum cover might be easier ?
tabetha