I went to cancun in march and stayed at km 16.5 and the beach was lovely there I'm looking at booking again in may at km4.5 are the beaches round that side as nice ??
In my opinion the beaches at Km 4.5 will not be as nice as Km 16.5. The north side (top of the 7) will have calmer water than the east side where you had stayed previously. If you do want to stay on the north side I think the best beaches are from the Presidente hotel to Kin-ha.
I have not stayed at the Barcelo Costa so I can not comment on the resort itself. I do know that area and in my opinion the beaches are not as nice as where you had stayed before.
