Here are my recommendations for each pane of the options:

• 3D View
  • Set Detail Area->Large if you have a decent graphics card, and run your screen at higher resolutions, this will improve visual detail as more of your window will be in full detail. Better for taking screenshots as well.
  • Try adjusting your fonts so the labels are readable, but not too distracting while browsing in GE.
  • If you use Google Earth to look at mountains, try setting Elevation Exaggeration to 1.5 or higher. This makes the 3D terrain have exaggerated heights. A setting of 2.0 would be twice as high. NOTE: the "Terrain" layer must be selected to see 3D terrain. Also, it is best to return the exaggeration to 1 if you are loading 3D buildings as they are sometimes set up with the default terrain setting.
  • I suggest you adjust the Overview Map size to what works for your screen size (tip: use Control-M (or Apple-M on the Mac) to bring up Overview Map in GE)
  • For other graphics performance tips:see this blog entry.
• Cache
  • GE 4 automatically optimizes this setting. You can adjust it, but I recommend leaving it alone unless you change the amount of memory on your system.
  • If you can afford the disk space, set your disk space cache to 2000 MB (2GB) - the maximum you can use - this will mean less downloading when viewing the same locations. Also, you can use GE offline better.
• Touring
  • Set the Fly-to/Tour Speed to a faster speed if you use GE a lot. This means you will fly more quickly when you double click or search for a location.
  • The Driving Directions Tour Options allow you to change the default camera view, speed, range, and angle while doing a flying tour of your driving directions.
• Navigation
  • Mouse Wheel Settings - I recommend adjusting the speed of the mouse wheel setting which will increase your control for zooming up and down in GE.
  • You can also invert the mouse wheel direction so it makes sense for you whether forward or backward zooms you down/up.
• General
  • Click "Show web results in external broswer" if you want (or don't want) to use an external browser when clicking on a link in a placemark description.

Now, go enjoy an improved experience with Google Earth 4!