Four is genuinely different, but there are now many, many exercises you can do with three which once mastered that makes four almost trivial once mastered.
The same is true with going from four to five, etc.
Here are the first few exercises, all for while juggling 3. In all of the following X and Y should be taken as all possible combinations of Left and Right.
* Throw really high from hand X, catch in hand Y, and carry on;
* Throw one from hand X, hold up the ball in hand not(X), catch ball back in hand X and carry on;
* Do that twice in a row with the same hand;
* Do it with the left, then immediately with the right;
* Do it with the right, then immediately the left;
Email me when you've done all those a minimum of 10 times each.
No link offhand but I recommend mastering 2-in-one-hand juggling (zero in the other) three different ways: columns, clockwise, and anti-clockwise. Learn to do each of those patterns such that you forget which hand was ever dominant to begin with and then you're ready to try 4 throws and catches of 4 balls.
Another good exercise is to juggle 3 balls in the usual fashion (cascade) and make a decision to go directly into juggling 2-in-one-hand and hold the other ball steady. Then resume your cascade and try it again, but this time, use the other hand to juggle 2.
These exercises are fun and give you a sense of accomplishment in and of themselves, but they also really help lay the groundwork for a solid four ball pattern.
When finally learning to juggle 4, some people prefer a synchronous columns(1) pattern or an asynchronous fountain(2) pattern. For the best results, try each until you forget which one you ever had a preference for in the first place.
The same is true with going from four to five, etc.