Get 4 now with all the expansions. It has got to be quite cheap now or wait until it goes on sale.
Buy 5 in 2 years or so when there comes a bundle of it and every expansion.
Both are great games, and 5 has the potential to be better imo, but it's lacking in content compared to 4 atm.
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I'm but a simple companion in the pack of gurus whereof SuperNiceDog is the pack leader!