Do you need to take all your luggage to Ireland just for 4 days.
If you are returning to the same hotel in London before your flight back to the US, you should be able to leave a bag or two with the hotel while you are in Ireland.
Alternatively, if you look at this link
http://www.gatwickairport.com/
Click on Airport Information and then on Left Luggage you will see this note
Both terminals have a left baggage facility for storing luggage or belongings for a number of hours or days. All items are security checked. Left baggage offices also offer competitive unaccompanied baggage rates and other services as well as selling luggage and travel accessories.
Charges are £5.50 per item per day (or part thereof).
Opening times:
North Terminal: 06:00-22:00
South Terminal: 24 hours
The service is operated by the Excess Baggage Company, call +44 (0)1293 50 20 14 for further details.