The GUI and interface handling has been greatly improved. An autolisten mode has been added, and 64 bit versions for Mac are now available as well.
LP10 4.0 features a whole set of "FlexFilters" (LP/HP/BP/Notch/Low- and High Shelf) which have the unique feature that the filter slopes can be continuously varied from 6 dB/Oct to 60 dB/Oct. Also for these new filters, the phase response can be freely tuned.
Price: The 4.0 upgrade is available for existing users with a 50% discount, and free for users who purchased LP10 3.0 after April 13th, 2011. For new users it's $29.