I'd say it's some of best science (?) fiction written, so I'm biased, but yes books 3 & 4 are worth it. It gets very bizarre and the ending has stuck with me for a long time.
I liked 1 & 2 a lot (I happened to read 2 before realising that 1 existed and that worked well for me).
3 & 4 I thought were quite poor.
3 & 4 are ... different and more conventional than 1 & 2. They can be disappointing but they do try to bring some closure to all the events.