Can a part 103 ultralight weigh 400kg fully loaded?
Part 103 limitations pertain to dry weight in takeoff configuration, maximum stall speed, maximum level flight speed, and maximum fuel capacity.  They do not directly address maximum gross takeoff weight.

However, weight affects stall speed, and I'm not certain how Part 103 defines its stall speed limit -- whether they specify at some minimum operating weight, or at "design weight" or some other value.  The complete regulation is available online, however, so you can read it yourself and determine if you'll be able to design for your maximum 400 kg GTOW.