[if I'm enrolled in a CE is the first 4 years in VIE written off as already served? /quote]
No - you just won't have served a VIE. Depending on the occupation, your age, and the requirements of the service, once the CE is completed, you could be released or offered other terms of service that are in place at that time (so technically you could go from a CE to a IE25). Some occupations have provided a CE after the VIE as they found that many people were reluctant to sign on for an IE25 after only 2-3 yrs in the military, much of which was spent on training.
Are there other career implications?
The main career implication is that you may have fewer years to accomplish your goals, attend training and gain experience.
if I go on as a VIE(4) and perform well enough to be offered CE(3), once I complete the CE will I be given the option to sign an IE to bring be to 19 years + 6 previously served(class A at 1:4 and Class B/C 1:1) or is the TOS and pensionable time completely seperate.
The IE 25 takes you to 25 years of service, which includes the previous Reg F time (VIE and CE). With the new superannuation rules, the 25 years for an unreduced annuity is not linked to terms of service. Technically, someone with extensive Cl B and Cl C service could get a 25 year unreduced annuity with only 2-3 years of Reg F service.
In order to get full details, you would need to sit down with the DAODs on TOS, and the CMP instructions, and the details of the superannuation act, with someone who has some experience and time to go over the details with you. If you are fortunate, and haven`t upset you Chief Clerk, they would be a good person to discuss the details of the various options you are looking at.