If you can get home each lunchtime then it shouldn't be a problem, 4 hours is the acceptable time to leave a lone dog, remember, they are pack animals so crave companionship, either with other dogs or humans, so to leave a dog for longer than this wouldn't be very fair. If you are getting a puppy, perhaps you can take a couple of weeks off to settle it into it's new routine. Be careful if you intend leaving it out in the garden all the time, Staffy's notoriously get 'dognapped' to be trained for fighting purposes, so just bear that in mind, Staffys make lovely pets they are very people oriented and they love children.
Good luck.