Breakdown Voltage of relay contacts in series
Quoted relay voltages are usually set by the switch-opening process.  Not by open-switch standoff voltage.

In other words, your 250V relay can tolerate the severe arcing which occurs whenever a 250V circuit is suddenly opened.  The same relay probably can handle far higher voltages before it would arc across, as long as the contacts weren't opening during the moment the 400V spike occurs.  (And even then, it might work OK)