Not only will you cause a crash if you change the Tx crystal without retuning the radio but you'll also cause your range to be diminished. That's why it is illegal here in the states (has been at least since everything went to narrow band in I think 1991). BTW, there is no advantage to using a 6 channel radio with a 4 channel receiver. There could be a benefit by using the programming in your radio or the model memory, but you can't use more channels than your receiver is able to hear.
So now I'll ask you the question that really gets at getting you a good, functional system. Why are you spending money on Ebay buying this older radio gear that you're now having to spend more money and time to make work when you could be investing in a good 2.4ghz system that has more features and is interference free? It may be more money to lay out all at once, but it will save you money in the long run.