I guess some GM might rule that retractable spurs (that typically come out of the knuckles) might be hard to combine with the muzzle of your hold-out cybergun (which also typically is fired out of the knuckles), but in that case you can also argue that your hold-out cybergun will be fired from a port hidden in the palm instead. Discuss it with your GM.
Rule wise, the limiting factor here is otherwise only the capacity of the arm. In a synthetic full arm you can for example install a heavy pistol while at the same time also installing retractable hand razors, but if you install a SMG then the cyberlimb no longer also have capacity for your retractable hand razors (time to switch to an obvious cyberarm with more capacity, perhaps?).
Edit: Ack..... I just now noticed that this question was specifically for SR4.
There might or might not be rules that prevent this in specifically SR4 (this I am not aware of)!