Re: 4 week old will only sleep on me!
At that age, my babies slept on me still quite a bit. Usually on the couch. I learned to just roll with it until they were about 6 weeks old then I started to put them in the basinett by the bed. I'd put the fan on for some white noise and music to drown out the other kids' noises. Sometimes they'd squak for a few minutes and then fall asleep. I tend to think that squaking (when it's not a cry/cry, but sounds more like a mild protest or baby cussing) helps rid them of energy and gets them more tired.
Also, my babies didn't love being on their backs, but liked this triangle thing that went on each side of them that placed them more on their side (I was too paranoid for tummy sleeping as well). That seemed to keep them asleep for longer.