To juggle 4 this way, start by holding 2 in each hand. Throw the first 2 as normal, except a bit higher. Throw both in your left over to your right. You can actually throw both of these over without resorting to passing. Practice this special throw until it feels comfortable. When they hit your right hand, throw them so one aligns over the other in the air. Catch them in succession and keep them going in the pattern. You need to keep track of which ones are being thrown together. It's possible to get them confused with the other 2 balls.
To juggle 5, start with 3 in the right, 2 in the left. Throw one up as normal, throw the next two together and pass the other 2 over. The idea is the same as before, you just have to follow two sets of 2 balls, and not confuse them with each other or the single ball.
For 6, each throw and catch involves two balls. I find it easier to start with 2 in the right and 4 in the left. I throw the first two up as normal, throw the next group over to the right, throw those and, finally, throw the last group over to the right and go from there. I find this much easier than starting with 4 in the right, where the first 2 need to be tossed while there are still 2 more in the same hand.
For 5, start with 3 in the right, 2 in the left. Throw the first 3 as in the 5 ball shower and start the group of 2 as before. As you can see, 6, 7 and 8 are the same idea, just more groups to keep track of. Starting 7 or 8 can be a problem, I still haven't found any convenient method.