Hey you! Why haven't you updated to the L5 engine? Logos philosophy is that the software should always be free... It's only the resources you must pay for. Some of the new features require new resources (I.e. data sets), so to use those features, you would need to purchase the core datasets, minimal crossgrade or base package. You do not need to do so, however, for the rest of the features to work just fine. When L5 was FIRST released, Logos limited its access to those who purchased a new base package, but always intended it to be available to all users. It has been available for some time now.
As long as you are running Windows Vista or better (i.e. not Windows XP), you are encouraged to update. One added benefit of updating... You can use the Logos 5 forum. This one is like a ghost town because most everyone has already updated. Now it's your turn! 
These are the enhancements you'll be getting with a free upgrade*:
- Updated Home Page, with better 'Go' layouts
- New Library features, including Custom Series and filter by collection
- New search features:
- Search helps
- Sympathetic highlighting
- Image search
- Root searches (only free if you have reverse-interlinears from L4)
- Share/Undelete/Sort documents
- Bibliographies
- Scripture Memory Tool
- Resource Previews
- Typography Settings
- Open/Sort Saved Guides
- Topic Guide
- Bible Facts Tool (combines Biblical People/Places/Things)
- Create your own Lectionaries and Bible Harmonies in Personal Books
- Translation section in information panel (only free if you have reverse-interlinears from L4)
- Syntactic data in Exegetical Guide (only free if you own Lexham SGNT. If not you'll need MC or Bronze+)
- Roots data in Bible Word Study (only free if you have reverse-interlinears from L4)
And these are the L5 enhancements you wouldn't have access to without purchasing new datasets*:
- Events in the Bible Facts Tool (requires CD, MC, or Starter+)
- Timeline (requires CD, MC, or Starter+)
- New 'Outlines' section in Passage Guide (CD, MC, or Bronze+)
- Sermon Starter Guide (requires CD, MC, or Bronze+)
- Referent section in information panel (requires CD, MC, or Silver+)
- Sense data in Exegetical Guide (requires MC, or Gold+)
- Update to Bible Word Study
- Phrases (requires CD, MC, or Bronze+)
- Sense (requires MC, or Gold+)
- Clause Search (requires MC, or Silver+)
- New visual filters:
- Timeline events (requires CD, MC, or Starter+)
- Speaker labels (requires CD, MC, or Silver+)
- Bible Sense Lexicon (requires MC, or Gold+)
But even though you won't get all the new features of L5 without paying for the new datasets, you can see that there are still a lot of useful enhancements that won't cost you a penny. So what are you waiting for?
* Thanks to Mark Barnes for compiling the list of features. This list was current as of August 8, 2013.