I used Personal Brain 3.0 for a while, downloaded the 4.X version sometime last month. While I really love the visuals and the mind mapping intuitiveness... I think the product is too expensive for the average individual. $99 for the Core edition if you are upgrading from 3.X. $149 for Core edition retail.
A less-pricey option is 3D Topicscape and it can import pba files exported by PB3 and PB4. Has the advantage of a 3D view. A big Plex in Brain is much easier to explore when imported and viewed in a 3D landscape.
It does need a pretty capable 3D graphics card, though.