I thoroughly enjoyed the first Shrek, even though (at that time) I was a bit skeptical about the new-fangled way of doing what appeared to be little more than beefed up cartoons.  I saw Shrek 2 and Shrek 3 with assorted children, and thought they were all right but weren't as good as the original.  I even spotted a continuity error in one of them - but shh, best not speak of such things here.
   So when I went with Boy 1 and Boy 3 (who have twenty actual years between them compressed into about one year when it comes to film preferences), I wasn't particulary expecting to think that Shrek 4 would be any better than numbers 2 and 3.  But I was wrong.
   This Shrek unashamedly assumes you know something about the characters from the previous films - without such knowledge, many of the situations and character twists would fall flat as a pancake.  The plot is a version of many another story, featuring a small change in the past which produces huge (and unwanted) changes in the future - but forunately our hero (Shrek) gets one chance to put everything right.  Which, of course, he eventually does.
   I thought the script was right on, the 3D animation was excellent. and the plot just sufficiently different to keep adults happy.  I recommend it.  One word of warning though:  number 3 son told me that he preferred Toy Story 3.  I haven't seen that, so I am unable to comment...