I use v4 at work and HD at home. For the most, HD has been serviceable for home use but after several years of getting by on HD, I'm planning on upgrading to v4.
While it's all spelled out on their website, these are the items I find most difficult to live without:
1) No stereo-to-mono conversion. For example, on my current project, I'm being asked to spot the majority of my sounds in mono.
2) No thesaurus support. This is a big one. I seem to get more returns from search queries in v4 than in HD. Makes it easier to find that specific sound that you're looking for.
3) Metadata support is limited. You can only edit the description field which is pretty gimp-y if you ask me. If you're planning on creating your own library and/or want to add your own metadata to existing sounds then you'll definitely want v4.
In a nutshell, if you're just starting out, these are things you can work around in one way or another. Eventually though once you start bringing in money, speed and an efficient workflow far outweigh the costs of the upgrade.