I have a submitted manuscript to IEEE transactions. The paper was under review for 4 months. Then, it has been with associate editor" for 2 week now. How this status can be explained?
The editor has received the reviews and s/he is in the process of judging them (sometimes maybe s/he will decide on a new review, but not likely) and communicating the decision to you. Sometimes this can take few weeks for various personal reasons (for example heavy work load on the editor). I expect that very soon you will receive the answer.
This status "With Associate Editor" means that the associate editor who is in charge of your manuscript has received the minimum number of reviews (usually 2) for your paper. This does not mean that he has received all the reviews he is expecting, some reviewers are invited or accept their invitations later than others (sometimes 2-3 weeks apart, even longer), or maybe the reviews he has received are conflicting and he is awaiting an additional review before making a decision.
The system in most Transactions lets you change the minimum number of reviews so that the status is "Under Review" rather than "with Associate Editor".
Also in most cases, the AE will receive an e-mail to notify him that the minimum number of reviews has been received and he must submit a decision within a couple of days / week.