You can have 8 feet wide beds, under certain circumstances. If you
plant pick-once veggies, like garlic, onion, beet, parsnip, and you
already have your mulch, compost and driplines in place, you will never
have to work on or around the plant again until harvest. And I do have
certain beds which are 5 feet wide and which I try to use that way.
But you have to plan. Some pick-once veggies actually need attention
during the season, for example cabbage and its green worms. For
pick-often veggies, like peas or tomatoes, 4 feet is best, if you have
access from both sides. Access on one side, 2.5 feet, no more. Most of
my beds have sides made with old sidings, so there is really no extra
distance. The few beds made with cinder blocks, yes, the block
thickness counts (or should count) in those 4 feet. You can always
plant an extra chard or carrot in the blocks holes.