The structure of 1,3-butadiene is ${\mathrm{CH}}_2=\mathrm{CH}-\mathrm{CH}={\mathrm{CH}}_2$and the distance between the central carbon atoms  is 1.46 A⁰ (instead of 1.54 A⁰). This is due to the

existence of non-octet resonance structures involving double bonding between the central carbon atoms
 

decrease in bond length when terminal double bonds exist

sp² -sp² overlapping

p-p overlapping