What is your favorite juniper?? I am looking for one approx 6-12 inches tall that reaches approx 4 ft in width, but am willing to vary if there are good ones that vary from those sizes.
That is very small. You might be able to get what you want out of one of the shore juniper cultivars.
If these grow there.
Otherwise, maybe something like Juniperus procumbens 'Nana'.
If it grows in your area, and does not blight off like it so often does here.
Right in the sales yard.
There are several junipers that meet your criteria. 'Blue Star", procumbens nana, 'Bar Harbor' are 3 that quickly come to mind. A local nursery should be able to suggest varieties adapted to your area.
'Blue Star' grows much taller than 12". Juniperus horizontalis cultivars can grow very wide.
Though wider than 4 feet in time, I'd say Juniperus horizontalis 'Mother Lode'.
'Blue Star' also is very intolerant of high moisture so it needs excellent drainage and preferably no clay soil. bboy is correct - gets taller than your specs. 'Bar Harbor' continues to expand much beyond 4'. Shore juniper varieties fail in KS so question whether they would work in MO.
How about some J. horizontalis var. that gets very heavily pruned every spring? Is there a procumbens or horizontalis that doesn't get beyond 4'? Love to know which one that would be. I could use that one!
Blue Star at more than 4'?! I'll take your words for it, but I've never seen one much more than 2' tall and my own which has been planted for 15+ years is about 20" at it highest point. All the online references I've checked indicate mature height of 2 to 3'. Could we be talking about two different clones?