Just a container to indicate the difference between these two families. Please add any other difference, I'm sure I will be missing some.
Features added to the A4U:
1) USB Module
2) CRC Module
3) 16 bit timers can be split in 2 8 bit timers
Other notes:
1) Greatly reduced errata on actual devices datasheets, of particular interest is the ADC.
Is the A4 a subset of the A4U?