Can you sit down and think objectively about your family as a whole? I'd try to look at the family as an outsider might. Is it possible that overall, he's the kid who gets a bit less attention than his siblings, for whatever reasons? Does he have a sibling who's a standout in some way (academics, sports, etc.) who gets attention for that, and/or a sibling who gets attention because that sibling needs more help with things somehow? It's possible that son "four of five" gets a bit (unintentionally!) lost in the mix if he has brothers who need more help or stand out more. This is NOT some fault of yours; it's just family dynamics, but it can result in a kid who does whatever he can to draw your attention--even if what he does gets your "negative attention." Negative attention (yelling, disciplining, etc.) is still a form of attention, period. It seems strange to an adult, who might think, "Why on earth would he annoy me to get my attention? Why not do nice things etc. to get it??" but to a child's mind, things don't work that way all the time.
So...think about whether this is not only about greed for stuff but about pushing your buttons enough that he gets yelled at and he's cool with being yelled at....On a certain level this may indeed be about "keeping up with the Joneses" and having and doing what other kids have and do. But I'd suggest the following:
Talk with him, as others suggested, about how this is both bad manners and hurts you; you feel like you're seen only a source of Stuff. Be gentle about the talk. Do not have the talk when he has just been asking for stuff; choose another, calmer time and have the talk alone or with dad there -- no brothers around. Don't make it about "you're so bad to do this" but about "When you do this, I feel...."
Then when he begs, ignore him. Or give him the cold stare: It can be very effective, with a child who is used to hearing you yell, to go VERY quiet instead of yelling! It can shock the kid a bit. He asks. He expects you to argue against him or even to yell. But instead, you stop and look at him. He will flounder and then you can say very quietly, "I'm waiting." He will figure out that it's time to stop talking.
And most of all, give him extra one-on-one attention when he's not expecting it and not asking for anything -- catch him being good, take him to do something together when he has not ever asked for it, etc. Be sure to focus on activities the family can do together, not on purchased objects.
Find things to praise him for: Schoolwork, something he did for a brother, a chore well done. You may have to go out of your way to do this and may praise him for things you feel he should just DO as a matter of course: "Why should I give him extra praise for taking out the trash? Geez, he's supposed to do that anyway!" But if he's pretty good about remembering it, and doesn't fuss about doing it, tell him you see that and appreciate it. This will sink in if you do it consistently.
Controlling your own reactions is tough, I know! But it's crucial here. That's why going very quiet and cool when your natural reaction is to yell and get heated about it can really get HIS attention. Talk to him like the near-teen he is and show you expect him to understand; go cold when he asks, not hot; and praise him and find ways to give him positive attention unrelated to the begging.