If U have AT3 presets U want to use in AT4 to make them work in AT4 U can 'Import Legacy Presets'. AT4 will load them and then U need to save them using AT4 'save as' function in the location of your choice.
Unfortunately U have to do this for each preset U want to keep, but it's a simple process. U need to know where the AT3 presets are located on your system and browse to that location in the box that opens when U click on 'Import Legacy Presets' . A browser box called 'Import Legacy Presets Root Folder' opens click on 'Browse' and a very familiar window opens, locate your AT3 preset and follow the instructions that pop up. Use AT4 'save as function' and U can rename or adjust the info about the preset.
( eg to locate a preset in AT3's Fender 65' Twin menu - factory location is normally -
... /Documents/IK/AT3/Presets/Fender/65' Twin/... - AT4 will remembers the AT3 location when U go back to load another.
(Full instructions see page 36 in AT4's user manual section 4.4 - Importing Legacy Presets')
(note: U can save them in a general location and them simply move them into the location in the drop down menu of AT4 U wish. I found this faster then putting in to the menu directly.)