And so we gots....H3C−CH=CH−CH3 versus H2C=CH−CH=CH2. The π−bond of the former creature is LOCALIZED between C2 and C3...and of course this could give rise to 2 geometric isomers, which are?
The π−bonds of the LATTER are delocalized over 4-centres...certainly addition of 1 equiv HX would give rise to 1,4−addition to some extent....