Your requirements and budget are going to make it difficult for you. In order to get a boat big enough for 4 adults overnight on for 4 days you are looking at something 35'. If the adults are two couples, and they are real friendly and you don't take much gear, then maybe a 30' boat would work. You'll have to decide how comfortable you need to be.
For 20K you're looking at old (late 70's / early 80's) Catalina, O'day, Newport, Pearson, Hunter, etc. 30' boats.
If you want something bigger, then you'll be looking at even older boats.
To be honest, if you really want to fit 4 adults in the cockpit, spend 4 day trips with 4 adults, and be competitive on the race course, you're looking at 30K easy.
Day To Remember, 1986 O'day 35 For Sale
Deep Blue C, 2002 C&C 110 For Sail
Mt. Sinai, NY
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