Try looking at BS EN 13414 Steel Wire Rope Slings - Safety. It gives working load limits for sling sets. It is pretty conservative for a four leg sling as it assumes only three legs take any load, but it will give you an idea.
Also supplier catalogues such as Crosby and Bridon contain lots of good stuff.
The LEEA that Steve directed you to have some publications such as the Lifting Engineers Handbook.
Another useful one is the International Rigging and Lifting Handbook by North Sea Lifting. Both of these books are called 'handbooks', but pocket books would be better as they are pretty small.
