Differences between atmega32u2 and atmega32u4?
Compared to the u2, the u4 has:


More RAM (2.5k vs. 1k)
More pins (48 vs. 32)
Analog-to-digital converters instead of just analog comparators
Low frequency crystal option instead of full swing crystal option
An I2C interface.


To answer this, I did a DigiKey search for each part number and compared the result tables.  This is easier than comparing the datasheets, although datasheets are the most accurate place to find information.  For example, the oscillator information in Digikey suggests that the u4 requires an external oscillator, while the u2 requires an internal oscillator.  This is not the case; the u4 ships with the external oscillator as the default and the u2 ships with the internal oscillator as a default, but both can use internal and external clock sources.