I don't know where you live, but there is always a lumber company some where that will deliver small loads for next to nothing or free.

Or if you knew of someone who was a contractor that had a ladder rack on the truck, that could haul them for you, you would be in luck.
I have not done construction work in about 2 years, but I would guess the weight of each 4 x 4 wet would be around 45 lbs.
Most luggage roof racks will not support 180 lbs of this type of weight.
There is no front or back bumper to tie off of and no way to support the weight - because the body of the car is unibody - hence there is no frame, the roof is a part of the frame and you can cause damage to the glass when you put a lot of weight on the roof if the roof buckles.

4 x 4 treated does not make very good mast material.
Use Aluminum or steel pipe.

If you look in Lowes or most electrical supply stores, you can find 1 1/2 semi rigid conduit for about $15.00 a section. They are threaded on both ends and has one coupler supplied with each joint of pipe.
I live in a high wind zone - winds in excess of 65 miles per a hour during storms and my Solorcon 99 antenna has sat at the top of 3 sections of pipe for 9 months now without any bending and it has no guy wires.
There is only 8 feet supported by brackets - so there is 22' of unsupported antenna mast holding up a big long antenna.
Find someone with a arc welder and have them design and build you some wall brackets, preferably out of used steel, ( 3/16" x 2.5" x 2.5" would hold up a lot of weight with guy wires) and purchase some muffler clamps and attach the conduit to the brackets and the brackets to the garage or shed or side of house and you will have a cheap mast for less then $150.00 if you do it right.