Personally I would go for 5.2. First of all that is the version (to be more precise 5.x) that is being constantly developed and it is going to be RedHat product as well. Due to that it is already much more advanced and will bring more features and enhancements over time.
4.x is not supported any more and as far as I know it is not developed either.
Unfortunately it is not compatible neither on process engine API nor process definition. Nevertheless if you consider it for new projects that is probably better choice.