High frequency, low noise clamp circuit
You specify that it "should not introduce additional noise", which rules out any possible components in the signal path, at least in a strict reading. 

So, okay, we could put an LDO regulator on the op-amp power supply rail(s) and reduce its supply voltage so that it is not 'railed' at 3.3V but does saturate under all conditions at 4V. If the LDO has the same or less noise than your current supply, there will be no additional noise, and if the op-amp functions well enough with a few hundred mV of headroom, the 1MHz signal may emerge unscathed. If the particular op-amp you have in mind does not quite allow that, it's often acceptable to reduce the allowable input swing to (say) 3.0V and lose a fraction of a bit of resolution. 

A clamp diode as @user50443 suggests is possible- that means you use something like a zener (say a TL431) to create a stiff supply that can sink current and that's a few hundred mV below your desired clamp voltage and use a diode (conventional switching diode or a small signal Schottky) to clamp the op-amp output through a relatively low-value resistor such as 50 or 75\$\Omega\$. Of course the forward voltage of a diode changes with temperature, so unless you compensate the supply you'll have some change in the clamp voltage, and Schottky diodes tend to be leaky so you'll have to do the sums as to allowable error. If you're only concerned about high frequency noise, the Schottky (eg. BAT54) +TL431 should be a good choice. 

There also "clamp" amplifier chips that are designed to clamp the output at a certain level. I've never found them adequate in performance, but if memory serves they might be okay for this kind of application if precision is not a big deal. 

Edit: Here is a simple clamp diode setup that will not affect signals below 3.3V significantly (you could notice a slight DC shift at high temperatures) and does not require a minimum load on your 3.3V supply. If you're sure the signal can never exceed 5V and you're sure that the load on your 3.3V supply will never be less than about 15mA (maybe you put a resistor on there) you can lose the TL431 and resistors and just connect the BAT54 to the 3.3V supply. 







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