Are you sure the flange that you have now isn't the "retaining ring" that you remember? Normally Ilex shutters have flanges that attach to the lensboard with screws, rather than retaining rings.
You could just get another Ilex 4 flange, but they're not very common.
www.mpex.com sells new Ilex 3 and 5 flahges, so you could ask them. Otherwise, a machinist like SK Grimes is your best bet.
I've got three boards with Ilex 5 flanges so I can move my big lens between the Sinar P, Gowland 8x10, and Korona 7x17.