Penta-4-en-1-yne!

 but-1-en-3-yne

1. First count the number of C-atoms. Here, No. of C-atoms = 5 (pent)

2. Locate the bonds other than single bond. Here the Triple bond is at 1 postion and double bond is at 4 position.

3. There is no functional group in this compound.

SO, the IUPAC name for this compound is:  4-penten-1-yne

but-3-yne-1-ene.

The double bond has greater priority than triple bond.

But-4-ene-1-yne

the answer guven by Prajwal  kr is appx correct. double bond is more preferred but no of carbon atoms are 5. so

pent-1-ene-4-yne

1,4-Butynediol. Skeletal formula. Ball-and-stick model. IUPAC name. But-2-yne-1,4-diol. Other names. Butynediol 2-Butyne-1,4-diol 1,4-Dihydroxy-2-butyne.