Are they all pretty much of a muchness then? Antigua/st Lucia/ Kitts / Barbados?
Sitting on my porcelain throne using glory-glory.co.uk mobile app
Barbados is a busier island than the others with a bit more to do although St Lucia isn't far behind. St Lucia is pretty hilly though and alot of the resorts are at the other side of the island with a 2 hour + transfer from the airport. I would recommend staying on the west coast in Barbados, primarily in St James as another poster mentioned. Nicest part of the island but easy access to get back to 'the gap and Oistins for some local fun.
Personally speaking, I wouldn't do AI in Barbados. Bed and breakfast or half board is good enough when you have restaurants like The Cliff and Cin Cin nearby but I can see why people do AI if it is to cover their drinks consumption.
Never been to Antigua so I can't comment properly but my missus works for Virgin and it is the only island in the region she is not very keen on.
St Kitts is very quiet, went there as part of a cruise and it is much quieter and less developed than any of the other 3 islands you mentioned.
I am probably a little biased given the fact my wife's family are from Barbados but to me it is the best island overall that I have visited in the Caribbean and I have been fortunate enough to visit quite a few places in that part of the world.