If the 4 unit cap is really needed for multiplayer balance at the expense of single-player campaign; I have a suggestion to allow both parties to reach a middle-ground. Invocation of Nehek's unlimited unit healing is part of the Vampire factions' SP balance and limiting it to 4 hurts how they play.
How about when these two spell are maxed (2 points) on the lord / heroes skill tree the unit cap of 4 is lifted and returned to unlimited.
This would stop these spells from being overpowered in the very early turns of campaign and allow it to still be capped in multiplayer.
The constant nerfs to units and abilities because of multiplayer balance is dragging down the fun in single-player campaign.
-Stop nerfing the Dark Elf Cold-One knights they are already next to useless in SP, but apparently too strong in MP. Just increase their multiplayer unit cost if they are so strong that is what it's for. Give them more HP(1000+) they have some of the lowest stats of all high tier cavalry and just die in legendary.
-Also undo the nerfs to the Ancient Salamander, they were fine as they were, or give them technology to allow them to be better again.