The default is a 3x3 grid if you don't specify the number of points. So you would want G29 u4 T V4 or G29 P4 T V4 depending on where you got your code from.
You might want to try the RCBugFix file at www.github.com/MarlinFirmware/Marlin and turn on Mesh Bed Leveling. That might help you with the not flat bed.