That's a better question for 'pbsd, but 4096 bit keys are infeasible to attack (outside of quantum computing) using any technique currently known to science.
I'm fond of saying that the thing that takes out RSA-2048 is likely to take out RSA altogether, so, over the medium term, look to Elliptic Curve instead of RSA. New systems should be designed with ECC instead of classical IFP/DLP crypto.
Is it unreasonable to call a thread that decidedly ignores quantum attacks worthless? I mean, absolutely speaking, given that quantum attacks really are or will be out there, is it not bad advice to suggest systems be built with quantum-vulnerable algos?
Assuming nothing changes, 4096-bit RSA has around 160 bits of security, roughly equivalent to 320-bit elliptic curves.
Depending on how Moore's law progresses and our understanding of the Universe, you may never need to change (ignoring all other possible ways your key can be acquired).
