d4s are harder than the other dice, but not THAT hard, just use both hands. Put your thumb against one edge, and your forefinger from that same hand against the edge perpendicular to the one your thumb's on. Then put your middle finger on the opposing surface from your thumb, and your forefinger from that hand against a corner. Then just spin.
It'll take a few tries to get it right, but it's not as hard as it looks.
D20s basically spin themselves. D12s too. D10s are natural tops. D8s are actually pretty hard to spin on their long axis, but spinning them on their sides is easy. D6s are tricky, but they do have a natural axis once you find it. D4s are definitely the hardest.
BTW, there are D20s out there that use a natural number progression where 1 is adjacent to 2, which is adjacent to 3, and so on. If you look at one of those D20s, you'll notice that there's always a "side" that has 20, 19, 18, 17 and 16 all together, converging on one corner. If you use the right motion, you can "spin" that D20 as you throw it and it greatly ups your chances of landing on a 16+. I used to use one of those dice in a D&D game I was playing in and eventually got the throw right, which pissed off the DM (who's one of my best friends) to no end. Once he figured it out, I stopped using the die, but me and my buddies had a huge laugh getting one over on this guy. He was very "DM-versus-Players", and it was pissing US off...it was nice to flip the tables.