As with every T, Y, or any other piece it directly abuts.  I found that 4" PVC has a bit greater diameter and I have a mitre saw, so I cut off a LOT of 2 1/2" lengths and used gorrila tape (great stuff!!) on each of the fittings or Gates to fit.  Can't tell you how many rounds to make, as every piece of PVC pipe seems to be a little different, but at least it is consistant for each piece of PVC.  For me it was somewhere between a round and a half, to half a round, to get a secure tight fit.  These take the abuse well that it takes to get everything tight.