EF 70-200mm f/2.8L IS II USM EF 20-200mm f4 IS EF 70-200mm f/2.8L USM EF 70-200mm f 4 USM
Image stabilization Up to 4 Stops Up to 4 Stops no IS no IS
One of the best things to have happened in photography. Earlier one was restricted to shoot hand held only in good lighting to avoid camera shake but with Image stabilization especially the new ones, one can get very good shots even at slower shutter speeds. Highly recommended to have. But if shoot only in the studio or controlled environment you can always go for the non IS versions and save the money for any other equipment. IS is very in unexpected unfavorable conditions when it is not possible to use a monopod etc.
Maximum aperture f2.8 constant f4 constant f2.8 constant f4 constant
When we look through the viewfinder we actually look through the maximum open aperture even though we may have set the aperture closed down. (It closes only when the shutter is released) So, we will see a brighter image through the viewfinder in case of  f2.8 lens compared to f4. Also f2.8 would have faster focusing in low light situations compared to f4. However, as the image quality at higher ISO is drastically improving compared to earlier cameras, one can even increase the ISO
Glass  Good glass is important to help control secondary spectrum aberrations
Fluorite element 1 1    
Ultra low dispersion 5 2 4 2
  23 elements in 19 groups 20 elements in 15 groups 18 elements in 15 groups  
Diaphragm blades   8    
Tripod collar        
  yes optional yes  
Filter thread        
    67mm 77  
  1.49 Kg 760 g 1.30 kg  
Minimum focusing distance 3.9' (1.2m)      
Focusing and zooming        
    with the Extender EF 1.4x II and 2x II teleconverters (requires f/8 AF sensitivity with EF 2x II)    
  f2.8 IS f4IS f 2.8 f4