Hi Mitch and welcome to the forum. There are 2 general rules to using power tools. If the piece is small it is usually better to take the piece to the tool (router table, drill press, table saw, etc). If the piece is large it is usually easier to take the tool to the piece (handheld router, drill, circular saw, etc). Your 4 x 4 could fall into either category.
A flush trim bit (bearing at the bottom end of the bit) is the wrong bit for cutting slots on a router table or free hand. A pattern but (bearing on the shaft of the bit) will but there is usually no advantage to using one. Quillman is correct in saying you might as well use a 1/2" straight bit and a guide, either a straight edge working handheld or a fence on the table. Personally I prefer to go handheld because the safest way to make a slot (at least a blind slot where there is still wood at both ends) is by using a plunge router to make it. If you are making an open slot (at least at one end) then a router table will also work.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.