Actually, since the users' required target specification could easily be met with 4 ACTs, a 5th ACT would only be a 'nice-to-have' which would also impose a ZFW penalty when empty. As it would be for most flights.
Now that the CC150T and A310MRTT have been fully certified, the cost in down time to include a 5th ACT would be quite significant. Certain systems would require some design modification and an extensive test programme would also be necessary to validate fuel transfer and CG envelope issues. Add to that the workload involved in revising all the paperwork and you will realise that it wouldn't be a simple modification. Although, of course, it could be done if there was an urgent operational imperative. It's just not particularly cost-effective.
The pods are Cobham Mission Equipment (FRL) 907Es.
Fuel quantity in tankers can sometimes seem paradoxical. For example, had it been possible to isolate the front cell of the VC10K2 fuselage tank, it would have been possible to load more fuel than otherwise. Because with even just a basic 4 person crew, at max fuel the CG would be nudging the front of the envelope. Often maximum Tailplane Incidence was needed, "6 and a bit TPI and a bit of a pull" would be briefed by the Air Eng, particularly on trips with several pax and a high fuel load. You would set as much TPI as the Take-Off Configuration Warning system would allow; even then, the take-off, flap retraction and initial climb could be very demanding! Particularly given the higher buffet speeds of the old K2 and the fact that aileron upset applied immediately the flap lever was at 'UP', rather than when the flaps passed 14.5 deg on the way up as in later VC10s. This required a LOT of anticipation and some very positive trimming by the pilot flying!
Sorted the men from the boys, did a heavy K2!
On trails we would often load 'trimmed max fuel' and it took a lot of juggling with pax, fuel and cargo to find the optimum solution. Our expert Air Engineers were masters of their craft though and always got it right. But if we'd been able to keep the front cell empty, we'd have been able to fill up to MTOW with a bunch of punters before reaching the CG limit.