Your message sounded as if you wouldn't mind having a rooster in the flock, so my post will start off with the assumption that you have intentions of keeping the lil fella. Yeah, it appears to be a cockerel.
This is a new chicken to your property. He needs to be quarantined to ensure good health. After quarantine, he needs to be in a look-don't-touch living arrangement with your pullets/hens, I've had great success by housing the new chick(s) in a large dog crate with a wire door (with wire sides too is a plus, but not mandatory). Then after ensuring that he's got places to duck into, under, behind, etc., once he's released into the run, places that the older birds cannot follow him will make for a smoother transition into the flock. Place food and water in his hidey-hole, safety zone that the girls can't get to as well.
(still) Assuming you want him in the flock, eventually fertilizing your eggs; you should know that fresh fertilized eggs taste JUST THE SAME as non fertile fresh eggs, and you'd never know the difference. While he's young, long before sexual maturity, incorporating him into the flock will provide for a smoother introduction into the flock, but within the coming year you'll start noticing what looks like squabbling between him and the ladies. You'll start seeing him chase, peck at, sometimes mount the girls aggressively. It is a rite of passage from cockerelhood to full fledged rooster. It's normal, and typically works itself out within a couple weeks. This is because as he sexually matures, he must gain the respect of the flock as leader, and no longer as subordinate. He is claiming his rightful place as 'Boss', and establishing the new pecking order.
If you're not committed to raising a rooster, on the fence about the issues then:
You need to consider if you want a rooster.
You need to decide what you will do if you do not want a rooster (or no longer want him, for that rare bird that just isn't a good fit, for whatever reason).
Would you find him a new home, or process him for your dinner table?
All of that said, I'd wait for the bird to either crow or lay an egg to be certain of gender; 4 weeks old is a bit soon for absolute certainty.