You'll need a header 51" for the opening with 2x4's each side supporting the header, (if a 2x4 wall), spec'd for your existing load. If this is an 8' wall height, you really cant go wrong by just putting in a 4x12 as a header.

A header is a simple beam that spans an opening in a wall. Even for a 48" opening, the support posts at each end of the header would create a greater point-load on the structure below than the distributed-load of the original bearing wall. In the best case solid blocking would be added in the floor framing and in the worst case a wall or beam below would need to be reinforced.

The size of the header would depend on the size of the load from floors and/or roofs above.

4x12 as a header

I know they're available somewhere. But I've never seen one nor had a designer spec such a monster; built-up headers are loads cheaper.

As macv says, think three dimensionally. Structural changes rarely cause a sudden collapse; everything just sags.

"4x12 as a header"
Pretty common out here. When 2x4 walls were used for exterior, they were very common. When codes mandated exterior walls to be min. 2x6, they are still used, just furred out. Mostly engineered header stock is used these days but is pricier. Engineers tend to frown on dimensional built up headers, doubling or tripling 2x's. I agree with that.

We dont know your foundation type, your roof layout, or any other details of your project. Best to get s reputable structural engineer in for a consult who can look up close and personal and give you advice rather than us generalizing or speculating.

The fact that you have not given us enough information to answer your question indicates that you do not understand the issues involved so you should not go forward without getting an opinion from someone like an engineer or architect who has seen the structure and is qualified to design the modification.

"Do we need a header or a beam for a 48' opening?"

Yes, you do. :)

Hum, we too would like to add a four foot walk out bay in the dining room end of our kitchen. It would be on an outside load bearing wall. The opening I invision for my bay would be approx 11 feet. Does anyone know if I would need to have a steel beam put in across the top, or just heavy two by 10-12? This small ranch is sitting on a wood daylight basement, and the extension would be approx 5 feet above ground level. A friend told us we could just add beams to the basement beams under the house, extend them out four feet without support? How did you come out with your header situation, and do you have any sagging in that area? We live were it is very cold in the winter ND, and don't want to tear in to anything unless were sure we can button it up quick before it gets cold.

I'll tell you the same thing I told the OP: The size of the header would depend on the size of the load from floors and/or roofs above. Without that information suggesting a beam size would only be a wild guess.

Assuming this is a one story house, the load is determined by which way the roof spans, the length of the rafters, and the snow load for your area. Because there are so many variables, the header might need to be a pair of 2x10's or a pair of 12" LVL's but I doubt you will need a steel beam unless there is too little space above the opening for LVLs.

Cantilevering out 4 ft is more difficult to design. You should get a structural engineer to determine how far back the new joists must span and how strong they must be. I suspect they will need to be LVLs each side of the existing joists.

In snow country it is dangerous to guess about anything that supports a roof.

Also a 4 ft cantaliever would be a no no where I am, the max they allow in my area is 2ft.
The 11 foot opening is definately one where an architect or engineer should be involved. especially with a cantilever. In my area a change like this you would need a permit for, especially since you're adding something visable, and the town would want to see the calc to support the speced header.

the 4ft header discussed above, although a structural engineer is best, any good contractor who looks at it should be able to spec it, if its a simple home design.

Richard M I a sure in the 14 years since the op started the thread that the issue has been resolved, one way or another.