Some kids don't learn to use the potty until 4, so by itself, this isn't unusual. Frustrating, sure, but totally within the realm of normal.
But I sense tremendous frustration in your post - your "HELP!!" in the title, the fact that he is screaming and crying, your feelings of failure, and your boyfriend's family pressuring you and criticizing you. That's a bad combination.
First of all, is your boyfriend the child's father? If so, then he needs to be working with you and his son. Either way, his family has exactly zero say in this. If you generally criticized by them for other things, then you need to find some firm one-liners that shut them down ("I've got this, thanks" or "he's on target with his milestones, thanks") and your boyfriend needs to get on board with you and take your side. If he's not doing that, you've got a big problem.
Meantime, you have a 4 year old who either isn't ready to potty train (which means that his brain isn't getting the "I have to go" sensation) or is so agitated by the battle up to this point that he's shutting down. Either way, you give potty use a break. You put him back in diapers or pull-ups and you say, "Whenever you're ready, honey, you let me know." Then stop talking about it, acting frustrated, rolling your eyes, or talking about the expense of diapers - whatever is part of your routine and frustration, you have to stop.
Have you looked at any good potty books? There are some wonderful ones and you can ask the children's librarian for recommendations. You should get a book or two for yourself that teach about developmental stages and parenting techniques for things like this, and share them with your boyfriend if he's part of the family. You can also get some books for your son. Read them as part of your regular reading, but not as a punishment or requirement while sitting in the bathroom. If you need a different type of potty seat, get one. Some kids like the kind with 2 steps and some handles, some just like the round insert that keeps them from falling in. Without knowing your sons specific fears (falling into the toilet, being separated from his pee or his poop, feeling abdominal cramping), it's hard to know which books to suggest. I'm sure other moms will have their favorites, and you can also search this question on Mamapedia to see what others have posted in the past.
You cannot win this battle - so you have to make it something other than a battle! He will learn this - he will! - I promise you. Read up, and whatever method you choose, stick with it - and everyone else sticks with it too. Kids get confused if there are too many people giving instructions. Is he in preschool or daycare? If so, find ou what terms they use and any books they have already used. Work together.
And be sure that you totally separate nighttime dryness from daytime dryness. Waking up from a deep sleep to go pee is a different developmental stage. This is not something you can train a child to do, and it's not something that a child fails to do because he's lazy or stubborn.