Are the other four players genuinely interested in learning the rules but are struggling with them? If that's the case, you might want to run a couple of sandbox games or quick one-night games until they get the hang of it. We all get rusty, especially if we aren't gaming on a regular schedule and it's been several weeks between games. But if they genuinely want to play, they will genuinely want to learn how...you won't need more than a couple of games of "practice" to get the hang of the basics. The rest will come naturally as the players learn from each other. It'll be bumpy for a few weeks, but then it will be smooth sailing.
But if they aren't genuinely interested in learning the rules? That's tougher. If these players just want to sit at the table, talk about their characters, and let other people "worry about the math" on their behalf, that's fine---if everyone else is fine with it. If not, you need to take them aside and have a chat. D&D isn't a very good spectator sport, and it can be really taxing on the other players and the DM to constantly have to carry them.
If that doesn't help things, and if other players are getting frustrated, you'll have to take a hard line with them.
The first thing you can do is insist that everyone buy their own copy of the PHB, and bring it to the table every time they play. This can help them feel more vested in the hobby...and it's a good way to weed out the players from the spectators.
The second (and most painful) thing you can do is stop helping them. Insist that everyone does their own math, looks up their own spells, and calls their own actions...and when they screw it up, tell them its wrong and try to again. Don't help them, or let others help them, even if the game has been at a standstill for twenty excruciating minutes. Be gentle about it, and be encouraging, but you gotta be firm. You have to drive the point home that learning (and using) the rules is non-negotiable, for all players.
I hope it doesn't come to that, because when people get frustrated their tempers get short, and things can get heated quickly. Be ready to calm people down, remind them it's a game and everyone is there to have fun, maybe call a break every now and then.
The last thing you can do is adapt your game. Maybe those four players aren't looking for a strict, by-the-rules style of game. Perhaps they are more interested in a looser, storytelling kind of experience. The rules should support the framework of the story, not the other way around, so if all the focus and detail on the rules is tripping up two-thirds of your players, maybe you should get rid of some of the rules.
EDIT: I was a little more short than I should have been earlier. [MENTION=1]Morrus[/MENTION] is right: people play this game for any number of reasons, and there is no "wrong" way to play it. The point is to have fun, always.