I don't know of any standard easels that frame 4 x 6. There may be a few specialty ones around of that size.
The traditional standard paper & easel sizes are
- 2-1/2 x 3-1/2
- 3-1/2 x 5
- 4 x5
- 5 x 7, &
- 8 x 10.
Notice that 5 x 7 cut in half gives 3-1/2 x 5, which cut in half gives you 2-1/2 x 3-1/2.
8x10 cut in half gives you 5 x 8 (almost 5 x 7), which cut in half gives you 4 x 5.
4 x 6 can't be had by cutting the standard size paper in halfs or quarters, so it's a bastard size. Like 14 x 17, which is no longer popular.
Since the easels aren't common, neither are the papers!
The 4x6 size came from color processing, and why they chose that over, say, 5x7, is a mystery to me, though it was probably an economic decision. 3-1/2 x 5 used to be the standard size for color prints from 35mm. I suspect they went to 4x6 to give a sense of a much bigger print without it really using much more paper.
Charlie