The 4 has better specs.
In 4k the 3 tops out at 15fps while the 4 does 30fps.
In 1080 the 3 does up to 60fps and the 4 does up to 120fps (better for slo-motion)
The 3 should be enough if you mainly plan to shoot in 1080.
They both work with the Solo. Me personally, I'd pay a little more for...