Not a great idea mate but it’s an easy fix!
Unless you have only just changed the oil in the last 1000 miles I would use it as an excuse to change the filter too.
This chapter of the manual explains how to do it:
http://www.zxrworld.co.uk/Manuals/H Model/ZXR400 H Chapter 6.pdf
You will need to invest in an oil filter wrench which you can easily get from your local dealer or here:
http://www.justoffbase.co.uk/s.nl/sc.2/ ... ter+wrench
Just remember to smear oil round the rubber seal of the new filter before you put it on and what ever you do don't over tighten the sump plug!
In fact if you don't have one a torque wrench is a great investment:
http://www.justoffbase.co.uk/Tool-Shop/ ... ches-Tools
One last thing, when re-filling the bike with oil make sure it is upright by ether using a rear paddock stand or a local passer by / missus / mate to hold the bike whilst you pour (don't go over the top marker on the inspection window) take the bike for a ride round the block then leave it to sit for a couple of minutes before re-checking the level.