Is there a reason why you want to keep the cells at 4C. As Natalia mentioned, primary cells are never processed at anything other than room temperature. WBC actually have drastic changes when you drop the temperature to 4C.

Thank you for your your help and suggestions. Let me give a little bit more detail about why we want to do a percoll at 4 degrees:

We are testing out digestion of tissues with a cold active protease that is active at 4 degrees. While cold shock will induce some gene expression, recent papers suggest that these genes will not be transcribed if the cells remain at 4 degrees throughout the entire protocol, hence my wanting to do a percoll at 4 degrees. Conversely, digestion of the tissues at 37 degrees with collagenase and then a room temperature percoll will induce a heat shock response, which is what we are trying to avoid.