Sophie,
Welcome to the forum. Never seen a bike rack for 4 bikes. I'm sure the PT can handle the load but it's hard to understand how your rack attaches and where the actual load of the 4 bikes will be on the car. You say "main support is on the rear window"
You mean by suction cups or leaning on the glass? It is tied down at the rook rack points and thru the hatch opening?
Maybe someone with more knowledge of bike racks will come along and give you a better answer.