Depends upon the variety and how wide it spreads. I have some vars that can be planted that closely, others that can't.

Dan

I use the California Wonder variety. I plant them 24 inches apart in all directions. So, in a 4x4 bed, you could plant four of these kind of pepper plants.

I thought about planting them closer together - like 18 inches - but it is good that I didn't because they grew quite bushy and used up all of that space.

Here is a link that might be useful: BsnTech Gardening Blog

I think I'll try 5 peppers in the 4x4's. Mostly will be planting sweet peppers
and Fat 'n' Sassy Bell Peppers in the beds. My hot peppers will be in
containers. bsntech, I use cinder blocks for some of my beds also. They
aren't pretty, but no one can see them in my back yard. I only do 1 brick
high and plant parsely and low growing flowers in the holes. I makes it
prettier. Thanks for info.
Linda

I grew Fat N' Sassy peppers last year in my 3x3 bed. I placed one per square foot and the production was outstanding! I also had Mucho Nacho jalapenos set at one per square foot. Again, great production on very healthy plants. No disease or pest issues. I'll place a link to my site. I have pictures of the Fat N' Sassy peppers posted on it from last season. I would highly recommend the 1' spacing with them. They performed beautifully.
Kim

Here is a link that might be useful: Kim's PA Vegetable and Flower Garden

I planted Bell Peppers (I think they were California wonders) 1 square foot apart but the beds were also a foot wide so there was nothing growing on two sides of it. Maybe that is why they grew so big. I also had one upside down in a hanging basket but that didnt work out very well. It was shaded by a tree. I dont think I am going to do that again. When thunderstorms came through, I had no place to put them inside except hanging on the back of the chair. That was a hassle!

Kim, I have a dumb question. In a 4 x 4 how many Fat 'n Sassy's
would that be?

Linda

Linda,
Sixteen plants. It worked out better than I had imagined for me. I think you'll really be happy with that spacing.
Kim