If you top the grass I would keep them off of it because fresh cut grass will cause bloat quicker than anything. Understocking shouldn't be an issue, just means they will probably be slightly pickier what they eat, if it is very good pasture can you section it off and graze smaller amounts at a time?
If you've introduced grass slowly then at 4 weeks they shouldn't get bloat, the rumen will have developed by then, what is in the pasture mix, does it contain rolled barley? If they are on good grass I can't see the need to feed anything except a little to keep them tame, my hand reared lambs are all out on good grass and definitely don't need any cake.