Nvidia 9400M vs Intel HD Graphics 4000
According to the benchmark comparison tool here, the HD 4000 has between 2x and 6x the benchmark scores of the 9400M. (How relevant the scores are to your gaming experience is yet to be seen. Some of these are synthetic benchmarks, so they may not be wholly relevant.) The list is ranked with faster cards first, and the HD 4000 is #181, while the two 9400Ms are at #302 and #310.

tl;dr: It seems that the Intel HD 4000 should be better than the Nvidia GeForce 9400M.

Note: to be able to select the 9400M in the list, you need to deselect the "Still available (not archived)" box to show old cards.