If you were looking for frequency modulated RF @ 4 Mhz you might well consider an Analog Devices AD9851 DDS solution (Faster X2 to 60 MHz) or the readily available from Ebay AD9850 assemblies. they require a 40 bit serial load SPI I think and can operate from nearly DC to 30 MHZ, both sine and square wave outputs too. While NCO's aren't new this one is interesting and at about 7 to 9 dollars an inexpensive way to go. There are a couple of sketches I've found for mine but not yet the time to test them... Nor a good frequency counter either... Yet.
Bob