Peter,
Our American cousins seem to think that it is normal to use an independant four jaw on the RT, and spend half their lives setting them up. A sure sign of masochism. Each to their own of course, everyone has their own way of doing things.
I personally use both three and four jaw self centring on my RT, preferring the four jaw most times. I have found thru experience that they do seem to be a little more accurate in the runout stakes. But also, I do tend to use the soft jaws a lot, and just bore out in situ with a milling cutter to get the accuracy I want.
The ideal is in fact having both 3 and 4 jaw self centring. That does allow a little flexibility when thru drilling, as you can most times, set up so that you are not drilling into one of the chuck jaws. That is why I went for my adaptable system of using Myford back plates for both lathe and RT.
So if I was to give you the advice, I would go with your idea of the four jaw self centring.
But don't forget to make yourself a centring spigot for the RT, it will save you hours of time in getting the chuck centred on the RT. If your RT has a 2MT central hole, just buy a cheap machineable MT blank, and machine it up to fit in the hole in the back of the chuck. There is one disadvantage to that though, you are limited to the length of parts you can hold in the chuck, but, on the plus side, if the part drops down the middle hole, you can usually get it out with a pearl picker or magnet rather than having to remove the whole RT to retrieve the part (hence the trick of stuffing a bit of paper down the central hole when depth isn't an issue).
If you need adapters or plates machining up, don't be afraid to ask.
John
