AATA Route 4, the Washtenaw Avenue bus, runs between Blake Transit Center and the Ypsilanti Transit Center. It is the most heavily traveled route in the AATA system, and thus runs the longest with the most stops. It runs every 30 minutes on the weekdays, every 15 minutes during peak times. The route from downtown Ypsilanti to downtown Ann Arbor takes 45 minutes.

There are three different route 4 routes (4A, 4B, and 4C) which make slightly different stops.

Notable stops include:

• Arborland
• Eastern Michigan University
• South University
• University of Michigan Hospital

Route 4 connects with AATA Route 7 and AATA Route 22 at Glencoe Crossing. A former park and ride lot at Arborland is no longer available.