Wow hate to see that. Most people are the other way, LOL. Alfalfa has deep roots and it will be hard to get rid of it. Ideally if it's a good stand of alfalfa I would just start over. Your best bet is to spray the whole field with roundup. This will kill it. Your other alternative is to plow it under. But unless you get a good deep plow alot of the alfalfa will come back.
After that you will need to work up the ground and then plant whatever pasture mix you want.
On 4 acres your cost won't be much. It's a little cheaper to buy it by the gallons than the stuff you get in the small containers. You can also get the generic which will be fine for what you are going to do.
If you can I would definitely drill it. You will get a better stand. If you can't drill it you will next to get the ground worked up really well. Then broadcast it and harrow it.
Before you bomb it with Roundup and lose you chance for organic certification for the next three years, why not visit The Stockman grass farmer at http://www.stockmangrassfarmer.com/sgf/index.html
The guys have a lot of insight into what you are trying to do.