I need to buy a 4 Ton compressor and need to know a good brand to choose and a reasonable price range for this unit. The estimate I have been given seems way out of line.
Is there a website to compare different brands of air conditioning units?

Well there are many different prices, odds are you need a new coil inside also. I would say installed any where from $2100 to $4000 depending on the brand and efficancy of the unit.

Get three or four prices from the locals. Estimates are usually free. Compare and weed out the ones you don't like. Remember the lowest price is not always the best deal.