Mudski, there is no longer 4" anything. Its either 100NB(nominal bore ) pipe which has differing wall thickness depending on the schedule or pressure rating, so it could be 3mm or 4.5 or 6 or 8,,,Wall thickness,, the point being it is a nominal dimension because it is a hot formed product, so internal dimension will vary a bit, but the OD will grow more than the ID - if that makes sense? Now there is 100mm tube which is a cold drawn product and is dimensionally accurate on the OD, and its ID will shrink as the wall thickness increases. So, if you measure 99.5mm on the OD its odds on to be 100mm tube. Nominal ID would then be twice whatever the wall thickness is. If the silicone tubing is still made to imperial, then there is your problem, and the only way out is to squash it with the hose clamp to get the seal. Hope this helps in some way, but not intended to do your head in!!!