06 Prius head gasket problem

I have an 06 Prius with 130,000 miles that I got around 8 months ago, everything has been running smoothly since i got it but last week it gave me the check engine light and it is very rough and shaking on startup. I took it to the local Toyota dealer for a diagnosis and they said that they did a pressure test and used a scope and that the 4th cylinder has coolant in it and that the engine needs to be replaced to the tune of $7k. The codes that I pulled up and that they also got were P0138,P0300,P0301,P0303 and P0304. The car did not overheat at any point and there is plenty of coolant in both containers and I am just looking for some guidance as to what to do next. Should I have an independent mechanic check it out? The dealer also said it would be about $4k to replace the head gasket Thank you for any help you guys can give me so I can make a good choice as to what to do next