Not a practitioner of the SFG method, just a gardener.
Location, location, location: taller crops need to get planted where
they will not shade out the other plants - unless of course you can
actually USE the shade, for say, greens during the heat of the season.
As already mentioned, and you noticed, access to the plant is
important. I have 3 tomato plants (OP) planted right smack-dab next
to each other - maybe 16inches?, but I can get to all sides of all 3.
Peppers are on the south side where, today, they are protected from a
ridiculous high wind by the wrapped tomoato cages. I have four
peppers, about 12" apart. If they seem to be too close later I may
pull every other one or try to TP.
Greens can get tucked in anywhere, so if you are pressed for space,
it's a bit silly to reserve a row just for lettuce. Put between
longer crops like cabbage or broccoli. Wide-row or SFG makes sense,
it's how Nature does gardening in the big 'ol world.
I like the idea breaking up the space a bit. 4ft is about the widest
you can reach across, so at a minimum, one additional walkway would be
good. It doesn't have to be any wider than a footpath - just enough
to stand in/walk on. Two paths would be ok - 3ft beds. The space you
are giving up to footpaths is not "lost" because it's an investment in
the health and care of your remaining crops.
Some varieties may not like being crowded more so than others, so
maybe your spacing was OK for other varieties - but it sounds like
maybe you need to increase by 25-35% esp. peppers. Last year I had
planted mine (baby bell, banana, jalepeno) about 16 inches apart, and
the plants themselves weren't giants but I got a decent crop of
Do some more research into wide rows and SFG, and good luck to you!