To expand on it as to 'why' CL4 is faster than CL9, here's 'roughly' how it breaks down - there's more to it than this, but it will explain the basics.
CL (or CAS Latency) is how many clock cycles a memory stick has to wait before it's ready for the next operation. So, a lower number is relatively faster than a higher number since it has to 'wait' less time between operations. So, a 1333mhz stick which has a CAS 4 timing will be capable of 1,333,000,000/4 operations per second - or 333,000,000 operations per second. A 1333mhz CAS9 stick would be capable of 1,333,000,000/9 - 148,000,000 ops per second (all these numbers are approximate). So - the CAS4 stick has more than twice the potential speed of the CAS9 sticks.
This is also a...