Or it could be one broker manually processing trades for four...

Or it could be one broker manually processing trades for four separate client accounts in succession. This is more likely in my opinion as the 9 to 13 second differences is probably the amount of time it takes the broker to input the details of each order into the system. Most of the algorithmic trading I see, the trades are usually spaced further apart. Have a look at the BMN trading on Friday for an example - lots of identical sized trades, but nearly all of them spaced well apart (with the exception of three that went through at exactly the same time). No instances of four in a row spaced a few seconds apart, and on the occasions two identicla sized trades are only a few seconds apart there is usually other trades in between.