You do NOT want to run these older sets on 5S NiMH, by the way: does NOT work any better, just will drain the battery just as fast: these older sets do not have voltage regulation internally like the newer ones that operate from 6 to 14,4 volt.
The advise from Pé Reivers back when he was still alive (he was amongst others importer for RcExl for our country and in general a worldwide known engine expert) was to run them on the lowest voltage practically possible, which is 4S NiMH, or 4,8 volt nominal.

A voltage regulator, set at 5,1V (for example) would also work very nicely, meaning you are NOT fixed to that separate 4S battery.
As said, I usually feed my ignitions straight from the receiver, and IF that receiver happens to operate on a different voltage, then a voltage regulator is a very viable and affordable option.