I have 4 different variable names that I want to collapse to a single column, though keep the identifying variable the same for the other variables.

Data Currently looks like this:

ID var1 var2 var3 var4
1 a1 a2 a3 a4
2 b1 b2 b3 b4
3 c1 c2 c3 c4

And I want to make it look like this:
ID newvar
1 a1
1 a2
1 a3
1 a4
2 b1
2 b2
2 b3
2 b4
3 c1
3 c2
3 c3
3 c4


THanks!