What's your gravity reading?
Measuring gravity is the ONLY reliable way to determine if primary fermentation is complete or not. Once gravity readings are steady for 3-4 days, you're ready to bottle.
If you've got a big beer, it may very well still be going strong after 4 weeks. If it was OG < 1.080 then you're probably complete. Operative word is probably.
Look at it this way: One more day won't hurt anything. Take a gravity reading now and take another one tomorrow afternoon. If they're the same, bottle it. If they're different, let it go a while longer.