What is the difference between ext3 & ext4 from a generic user's perspective

A few years ago I'd say stick with ext3 but nowadays ext4 is better. A recent (May 16, 2011) round up from thegeekstuff.com sums it up rather nicely:

• Supports huge individual file size and overall file system size.
• Maximum individual file size can be from 16 GB to 16 TB
• Overall maximum ext4 file system size is 1 EB (exabyte). 1 EB = 1024 PB (petabyte). 1 PB = 1024 TB (terabyte).
• A directory can contain a maximum of 64,000 subdirectories (as opposed to 32,000 in ext3)
• You can also mount an existing ext3 fs as ext4 fs (without having to upgrade it).
• Several other new features are introduced in ext4: multiblock allocation, delayed allocation, journal checksum, fast fsck, etc. All you need to know is that these new features have improved the performance and reliability of the filesystem when compared to ext3.
• In ext4, you also have the option of turning the journaling feature off.

A very good comparison from 2009 on linuxologist.com has a graph about write performance with 4 GB:

and also has some other information on ext4.

I myself have had zero problems with ext4.

If you need to share a partition with Windows you will need ext3. I myself have given up on Windows so I have everything set to ext4 and use a USB stick to transfer data to a Windows machine (mainly at work).

What you can do for a setup is the following:

/ ext4
swap
/home  ext4
/datapartiton_with_windows ext3

Then you can make the last one a shared partition for both OS's.

The only reasons I can think of at the current time not to use ext4 are compatibility with older systems, and, last time I tried it, ext4 had problems with losing data when used on flash media (ie. Thumb drives et cetra.) That second one I haven't tested in about six months, so it may have been fixed. Otherwise, the performance and reliability are large improvements over ext3.