The titles take steps forwards and backwards in certain areas. 3 allowed for an almost-seamless open neighborhood, allowed you to edit your neighborhood (both in and out of game), had a colour wheel for item customisation and allowed custom patterns, but nickel and dimed the shit out of you and was poorly optimized.

4 somewhat pulled back on the nickel and diming, offers more meaningful gameplay items, does more interesting things with its worlds (secret areas of the world) and is better optimized, at the cost of limited lots, limited worlds and limited customisation, alongside a lack of cohesion in the world (you'll often see townies walking outside your house during school/ work hours) and less customisation with the removal of custom patterns and the colour wheel.