Ben,
There are several ways to do it, here is an example:
In[1]:= (* with assignment *)
v = a b c h ;
a = 1 ; b = 2 ; c = 3 ; h = 4 ;
v
Out[3]= 24
In[107]:= (* with rules *)
Clear[v, a, b, c, h] ;
v = a b c h ;
v /. {a -> 1, b -> 2, c -> 3, h -> 4}
Out[109]= 24
In[4]:= (* with function *)
Clear[v] ;
v[a_, b_, c_, h_] := a b c h ;
v[1, 2, 3, 4]
Out[6]= 24
In[7]:= (* with pure function *)
Clear[v] ;
v = #1 #2 #3 #4 &;
v[1, 2, 3, 4]
Out[9]= 24
Also see this, where useful basic concepts are explained.
In general you can get all you answers from Mathematica documentation.
I.M.