Also depends on time of year. I did a Tuesday in October and was able to walk onto most everything in both parks. If you go during an off-season weekend, Disneyland (the original park) can get crowded. I've gone on Saturdays in March two years running and couldn't believe the crowds--of course, this last March Rivers of America attractions and Critter Country were closed.
DCA only gets bad during peak season, like Christmas vacation. Disneyland gets dangerously nasty crowd-wise during Christmas and Easter breaks, and has some packed days during summer.
At minimum, if park hours are kind--meaning 9:00 to 9:00 for DCA and 9:00 to 11:00 or later for Disneyland--and if you have the kind of pass that lets you go from park to park, you can do both parks in two days if you max out the hours.
If you have the ability and time to do it at a more leisurely pace, you should be able to see everything w/in 3 days. Four days will give you the ability to nap, check out Downtown Disney and the 3 Disney hotels, even re-ride a few faves. But it all depends on the park hours. Off season weekdays has parks closing by 8:00pm at the latest.
Hope this helps.