Quantum computers can't solve arbitrarily large factirisation problems. The signal gets weaker the largers the numbers get to the point where factoring a 4096 bit number is not going to happen.
Real quantum computers don't exist. If one did exist it would need to factor numbers much larger than 4096 bits efficiently for it to be considered a real quantum computer.
And all true scotsman wear kilts. We consider 8bit computers real computers even though they can't run windows 8. The same can be said for quantum computers, a 64qubit quibit quantum CPU could be vary useful even if it can't factor 4096bit numbers.
PS: We have solid state 4bit quantum computers right now. They may not be useful for much, but that's in part because we have spent hundreds of billions of dollars perfecting classical computers.
