Hi Christof,
A No. 4-1/2 weighs about 50% more than a No. 4 and is even slightly heavier than the longer No. 5. With its short sole able to follow slight undulations in the board and its 2-3/8" wide blade it the supreme tool for finish planing large areas. In use it's wide sole and low centre of gravity give it a feeling of solid stability.
The 5 is noticably narrower, great for its intended purpose of taking deeper cuts with a more heavily cambered blade during the earlier stages of preparation. Traditionally used across the grain as well as along it, it feels more nimble in use than the 4-1/2.
The pitches are the same, 45 degrees, if you want a steeper pitch then a small back bevel on the iron achieves exactly the same result. If you want to switch between the two you can use two irons and swap them out as necessary.
As for the price, the bigger cutting iron probably accounts for most of it as these are quite expensive to make, require a bigger frog and therefore take up more machining time.
Worth it? It depends what you want to do with the plane, if you want to produce a flawless surface on an almost finished board, the 4-1/2 is an excellent choice, if you want to remove bumps and wind from a board before flattening it with a try plane, the No. 5 is a better tool.
A popular approach when used in conjunction with surface planers and thicknessers is to use a No. 5 or 5-1/2 set up as a smoother and use it to correct the settling that occurs after boards have been machined and acclimatised to their new dimensions. If you're work is predominantly mechanised this might be worth considering.
Hope this helps,
