We get this question A LOT and the answer is five. If you want 1-gallons and you are substituting 4” pots, use the same number of plants. A 1-gallon and a 4” pot of the same plant will grow to the same size when in the garden. It is not a dwarf or smaller because it is in a 4” pot. So, prevent over planting and overcrowding and use the same number of plants. Generally with many of the perennials, you cannot tell what was a 1-gallon plant and what was a 4” plant after six weeks of growth in the garden. 4" plants are easier to plant (smaller holes to dig!) and they save you money because they are cheaper. Get more jobs done in one year; the quicker you complete a job, the quicker you can begin another one.
*Remember, wider spacing means that the individual plants will be of better shape because then they develop unhampered and they are easier to prune.