I dint see your post properly. Assumed its a DT modulator. For a continuous time modulator, the integrator bandwidth is much lower, about 0.7 to 1.3 times the sampling rate. So you need UGfreq about 640 MHz.
Simplest 4-bit quantizer would be a 4 bit flash ADC.
The feedback DAC needs to be carefully designed for a CT modulator. The DAC response is part of the loop filter response. You need a 4 bit DAC instead of a one bit DAC.
There's a nice paper just published at European Solid state Circuits conference which talks about multi bit quantizer CT sigma delta ADC.
S. Pavan, N. Krishnapura, R. Pandarinathan, and P. Sankar, "A 90 µW 15 bit Σ-Δ for digital audio'', IEEE Proc. European Solid State Circuits Conference, 2007, pp. 198-201.
Let me know if you cant find it.