If you do not have the windows TA installed, you can do something quick and dirty, like:
That will find your event ID, but to get the user name, you will need a fairly complex regex query using the rex command, because there are two "Account Name:"'s in the log, and you are probably looking for the second one. If you can use the Windows TA, it would probably be best to use that. Or follow gpradeepkumarreddy's advice, and just use:
index=<your_index> "EventCode=4738" "Account Name:*?<user_name>"