A 4 cell NiMH battery is driven to about 5 1/2 volts when fully charged. That's where the IBEC's Vreg is typically set so there's practically no voltage to drop. As the battery gets used and volts drop, the IBEC's built in Vreg does nothing. The CDI sees what it sees which is often lower than it needs to work right.
A 5 cell reduces the problem but it also may not have enough voltage at some point during a session, to drive the IBEC Vreg properly. A LiFe 2 cell is a better solution