As the names suggest, four-post car lifts have four posts, one at each corner, while two-post car lifts have just two at the rear. The two-post design requires a sturdier base to balance the weight of the car and a stronger platform to ensure the vehicle stays level.
A four-post car lift can lift almost 1 tonne more weight than the two-post models, so if you are lifting anything over 2500kg a four-post lift is the sturdier option. However, most cars, particularly classic cars and luxury cars are well under the 2500kg weight limit.