If you buy a ProntoNG model, this feature was added to the last set of updates for both the monochrome and color series.
With a classic you'll probably have to resort to constructing long Pronto Hex strings and as Lyndel mentioned, there should be a number of threads on the subject. You might still need the assistance of an expert like John Fine to put together your specific code if it's a bit different than the examples in the threads.
I'm also not sure if you can extend the string long enough to last for four seconds. That will probably depend on the Pronto software (firmware) more than anything as well as the protocol being used by the device you want to control (this caveat is probably mentioned in most of the matching threads).
I've also read people having success with very fast repeating, which you can also construct with macro sequences and possibly modified Pronto Hex. Again, success depends on the abilities of the Pronto and definitely the acceptance of the protocol and device to this type of construction. This method has also been discussed quite a bit for a number of devices.
