It should work at 6 MHz and possibly even somewhat above 6 and below 4. In any case, the circuit is very simple, so the best thing is: hands-on, try it yourself!!
I do not mean to be harsh, on the contrary! Though, you DO realize that you did
not answer the question at hand, I hope.. I will interpret your answer as "Yeah,
just change the crystal and it should work (in the region of 4-6MHz)", taking it
one step further and adding "The values of the resistors and the capacitor do
not alter the output frequency"
I really have considered your "hands-on" approach, but I don't have any kind
of instrument that can measure frequencies of 6MHz. Were that the case, I'd
probably have answered the question myself.
Thank you for your time nevertheless, but since I don't consider my question
answered if anyone has anything to add, it would be welcome!