This is the default behaviour for Riverblossom Hills (and it's mentioned in the Seasons manual). Your neighbourhoods should start with all four.
What, me read??
S'okay. Why Maxis chose to make the new neighborhood that shipped with the game anything other than the "normal" four seasons is beyond me. It's caused no end of confusion and loads of whiny freaked out brats on the BBS. Of course, they also made a borked family whose twin's parents are either two women or a woman and a dog dependant on where you drop them, so I guess it's not surprising.