In my opinion Greatswords are fine as they are. Giving them basic strength 4 is way too much... as well as giving them shields (Reiksguard foot knights)...
Chaos Warriors are influenced by the Chaos Power! Evil entities summoning troops to 'spread their dark will'... Well... You expect remarkable strength and endurance (toughness and high armour-save that is!) from these troops! Greatswords are heroic figures giving their best selves in the battlefields...but still, they're only humans
For this reason, I like the idea of fathsie, 'confering their leadership to other units within 6 inches'. 'Other units inspired by their presence' is a good explanation (even though it's similar to the 'detachments use their parent's unit leadership' rule)
I'd suggest that
upgrading only one greatsword unit to 'Carrorburg Greatswords' (for e.g.) giving them +1S and counts as a rare choice instead, would be a fair rule too..
Finally,
I don't like the idea for Greatswords to be skirmishers...just think it this way:
On the battlefield the courage is constantly tested. Man has to overcome the fear of death (not as easy task as it sounds
) and in order to push yourself fighting on even against impossible odds, you need to rely on your comrade. And that's what greatswords are all about: Companionship overcoming the terror of loss..
On the other hand, to achieve this, you need strictly military discipline and constant train! Greatswords are the absolute achievement and feature that state troops can display and the true pride for any professional army!! You expect from such a unit to follow pure military attitude! And what's the best way to do this but holding your ranks when moving and marching? (Can you imagine a world were Greatswords are 'skirmishers', while Evil Chaos Warriors and Goblins move in ranks like a disciplined army?!
)