I have used a grobag for Charlie since he was a month old and I agree, at 4 months I would go straight to a 6-18 month bag. I had a 2.5TOG and a 1TOG and between these he got through winter, spring and autumn, with varying the warmth of the clothes he wore underneath (I never used a blanket with him). I don't bother putting him in a sleeping bag in the height of summer at all.

I moved to a bigger bag when Charlie was 15 months old, so he got ten months' wear out of those two before getting a tad big for them. I've never heard of the dreambag either, so can't help you with that one.