how to convert spindle rotation to 4th axis ?

hello
ok, i need to install a 4th axis on my machine
i dont want to make it as rotating axis,

now the problem i that i want to replace the router with a much stronger and heavier one, that i do not want the whole assembly to be rotated by the stepper and hold it into position for it to work, i just dont think it will be stable enought.

is there any mechincal idea or plans that i can use to transfer the spindle rotation to the endmill without actually moving the router itself ?

thanks

There's this thing...

http://www.5-axis.com/

And from what I've heard in the past, it's VERY expensive. Tens of thousands of bucks USD.

Why don't you use a rotary table 4th axis? Much less $.