in my opinion the 534/16 (IV), the very last model, is much better. it's smaller, lighter, the rangefinder is better, film advance is less subject to overlap of frames. I bought 3 times a 533/16 from ebay, every time I asked the seller about the light meter, they all said that it was working fine and 3 times it wasn't. so I had 2 more tries with the 534/16. the first had a non working light meter as well (advertised as working) and the second, the one I kept, is kind of working, ways off the track, only useable in bright light (3 stops off the track).
nevertheless the IV is a wonderful camera. if you hold both models in your hands, the older one is a heavy, sturdy, kind of pre-war machine, whereas the newer model is a modern camera. I also bought a 531/16 (III), the same camera without light meter. it's a bit smaller, slightly lighter and quite less expensive. if you carry a tiny light meter (or fix it to the accessory shoe), you may have better results depending on your meter.