My associate editor cannot contact the 2nd reviewer
You cannot suggest the editor do something - the editor doesn't need your advice on how to do his or her job. What you can do is ask for a decision. Say something like, "it's been four months since I submitted this revision. When can I expect a decision?" The aim isn't to get the AE to make a decision, but rather to look at your paper and its status. The AE should understand that four months is a long time to wait, and that he or she should decide what to do next instead of continue to wait for reviewer #2.

As for probable outcomes, there's a good chance you might have to wait longer. From your description, reviewer #2 was the one that suggested major revisions, which is why the AE is particularly concerned about hearing back from reviewer #2. If the AE is unable to contact the reviewer, then the most likely decisions are 1) accept or 2) invite more reviewers. If the AE trusts reviewers 1 &amp; 3, (s)he is more likely to pick option 1; otherwise option 2 might be the choice and you might have to wait another 4+ weeks.