Does the 1851 Peel census give your 4x Grandfathers age?
Another option to consider when you visit the National Archive is whether he was employed by the Board of Ordnance(responsible for the supply of guns etc to the army.)Either way he may be listed as a pensioner.There is an excellent guide to Army Records for Family Historians published by the PRO and available in the bookshop at the Records Office at Kew