I would think that these results depend a lot on what kind of a machine your testing (single cpu vs. lots of CPU's) and the IO subsystem. WHen you restore 4 databases in parallel you create a lot more IO and write queues are more likely to appear than with just one restore process.
I think you should monitor your system and investigate what is the bottleneck.
[font="Verdana"]Markus Bohse[/font]