The Ronin Hayate Z70 from SR2 appears during the second cutscene in the mission where you go save Johnny Gat.
On another note, Vehicles from previous Saints Row games like the Keystone, Wakazashi and Blaze for example appear in the PAX demo for Saints Row 4, but don't appear in the final release which really sucks because i still like driving in this game even if you do have superpowers that let you run fast.