Anecdotal evidence seems to suggest your odds are better when using either 2x 4-star or 2x 3-star than when using a 4-star and a 3-star together.
People have fused Epics from 2x 3-star in the past. 2x 3-star has 2-5 star as results, but the materials for the 3-stars are endlessly farmable.
If waiting doesn't help, try (force) closing KnD and see if you get the gems when you restart KnD.