So what are you going to do when you have 4 fish in the DT and they have ich and are in danger of dying? You can't take them out to treat them since they won't all fit in a 10gal tank. You can't treat them with the live rock in the DT. You'll have to take out all your LR, put it in another container with a powerhead and heater, then treat the DT or use hypo. I'm not trying to give you a hard time, I'm just saying it's easier in the long run, less trouble to just have a QT. A small investment from the beginning and a little extra work will save you a lot of problems down the road.
It's up to you what you do, they're your fish, your money. But I'm telling you from reading and from personal experience, you can't buy fish and put them directly in your DT and not get ich.