Why not make 2-4AM charged at 5000MB/unit and 4-6 at 1GB/unit
That should be enough to split the usage a bit more. 500MB/unit is enough that most people will not change, but some will want to get the higher usage and change to 4AM splitting usage a bit more than it is now.
( I may have got the times and usage amounts wrong, but you get the idea. Make 2AM cheap enough that most people will be happy to do big downloads then but have an even cheaper option at some other time so that the usage gets split up a little more.