How many chain links do I need for my chain
Generally, you would buy a chain that is "long enough" and then shorten it to the correct length. If you're adding 4 extra teeth, then presumably you would need at least 2 extra links from what you currently have. Look at Sheldon Brown's chain length article for determining the correct length.


  The best technique for setting chain length is to thread the chain onto the large/large combination, without running it through the rear derailer. Mesh the two ends on to the large chainwheel so that one complete link (one inch, -- one inner and one outer half-link) overlaps. In almost all cases, this will give the optimum length.