The price difference between Poser 4/Artist and Poser 5 is only about $20, depending upon where you shop.

The majority of products on the market (and in Free Stuff) are for P4, and will usually work fine in P5 with minimal adjustment. Many P5 products will work in P4, also, the major exceptions being dynamic cloth and hair, and anything that depends exclusively upon P5's procedural materials.

Michael and Victoria are highly recommended; they have all manner of physique morphs built-in. The older figures bundled with Poser 4 (and the newer figures included in P5) are more limited in terms of morphs, and don't bend/pose as realistically.

Will you regret not getting the latest version? That depends upon your needs. P5's dynamic cloth feature can produce some very cool results that are almost impossible to achieve with P4's posable clothing alone. P5 can also create realistic reflective surfaces. It fixes the crosstalk bug that occurs when you have multiple figures in a scene, has better library management, more powerful material options, etc. It's also slower than P4, and many people have experienced stability problems.