J, <s>23</s> 22 chars
=

       (+/ .*&2 3 4 1@(+<./))

Example:

       test =. 3 5 4 3
       (+/ .*&2 3 4 1@(+<./)) test
    70
   
[Try it here.][1]

(23 long explicit function definition: `v=:3 :'+/+/\.3|.y+<./y'`)


  [1]: http://tryj.tk/