As the angle gets bigger, the edge gets finer. This means that unless the metal gets harder, the edge becomes more delicate and can be more easily nicked and worn. The harder the metal is, the more expensive the shear will be.
From the TOOLS page the #1 shear is being used on dry or coarse hair free hair so the edge needs to be stronger to reduce wear and "rougher" so the hair doesn't slide out.
For the #2 shear the edge still needs some strength but since you will be using this shear on hair you are holding between your fingers so it can't move, it can be a finer smoother edge so you get less stress on your hands.
The #3 shear will be used in ways that don't necessarily require the shear to be closed to cut but most of the time the hair is still being held so the blade needs to be very fine and very smooth so it slices the hair razor like as it moves past it.
If you have big hands then you may be able to use one shear for both #2 and #4.
If you plan to pass up the good money men will pay if you can do a good mustache trim then you may also be able to have one shear for #1 and #2.
The #2 shear can be specialty sharpened so it will do slide, slice, and slither cutting but if you plan to do that you need to buy a shear with the hardness of a #3 shear.