The schematic for a function generator will tell you what you want to know.

Start with the application schematic for a MAX038 or whatever it's called. It's a function generator chip.

If you need only arbitrary digital waveforms, you can use an oscillator driving a counter chain, in turn driving a RAM loaded from an intelligent source of some sort, and the output of the RAM used to load a PISO shift register. That's essentially what a monochrome video circuit does. If you need a variable frequency, then you drive the counter chain with a variable oscillator or perhaps with a frequency synthesizer.

If you want to use programmable logic, then a XILINX Spartan or Virtex FPGA will provide DLL's with which you can multiply up an external oscillator of whatever frequency you desire to produce the rate you need.

You have lots of options.

... and, BTW, 4-5 MHz doesn't even begin to approach "fast."

RE