im is right, but I'll add that this is likely /trans/ CH3-CH2-CH=CH-CHO.
the reason is, you said you see 6 peaks from the proton on C3, and I bet the intensities of that multiplet are 1:2:1:1:2:1, which is a doublet of triplets with triplet splitting 6-7Hz (CH2-CH) and the doublet splitting ~15Hz (trans CH=CH). A cis isomer would have had cis coupling closer to 8 Hz which would mess with your 7Hz aliphatic coupling giving you something that looks closer to a quartet. But I don't have a reference spectrum of the cis isomer to look at, so I can't be sure (those cis couplings can range anywhere from 0 to 12 hz, after all)
But the database chemical shifts for the trans isomer match your description :)