update tags.js to make nodes at indexes 3 and 4 unelectable

In tags.js, during some of the write concern tests, it is possible for the previously isolated node 4 to stand for election (see test case "partitions: [0-1-2] [1-4] [3-4]"), causing an unintended change in primary status. Setting the priority of one of the connected nodes to 4 will help to prevent an unexpected election from happening.