The stainless steel probe of the thermometer is an excellent conductor of heat and so the area immediately around the probe itself will heat up quicker than the main body of the meat itself. With thinner cuts of meat like Brisket the risk of contamination from pushing in a single probe just as it goes into the hot smoker is no more than you will get by simply trimming underneath the flap with a knife and letting it flap back before cooking. Even with thicker cuts of meat like rolled pork shoulder, even when it is un-rolled before cooking (which I know a lot of us do) the butcher has already done more introduce bacteria into the centre of the joint than a single probe will ever do.
Yes there is a theoretical risk of contamination from the probe however when put in context with all of the other more likely contamination points in the preparation process that have already occurred, that risk is negligible. I am very aware of food safely and supervise it myself but in the environments we are cooking and with good hygiene this is really a bit of a non-issue - unless the probe is dirty or has been inserted hours before the meat is put into the hot smoker.
There is nothing wrong with waiting the 4 hours, and I know people on here do, however it is really about assessing the levels of the actual risks involved.