You could add decorative end panels to the two base units to match the existing, will look nicer and be consistent with the rest of the kitchen, fixing from the inside and raised slightly off the floor.
With the other two panels, as advised, either undercut, or flush cut and beading. Angle brackets can be used to fix to wall (from the inside of the panel), and likewise on the floor as Azlog suggests (or a variation of it without the angle brackets) again on the inside of the panel.
I get (got) a neater finish, as a DIYER, with a sharp handsaw, cutting carefully, than I can with a multi tool.
Cheers,
Blup