See this question for reference.
I've got most of my problem figured out. I've split up the one large table into four smaller tables. I'm now moving the data from the four source tables to four target tables. Each in their own mapping. In Workflow Manager, each mapping is a separate session, inside of its own workflow. And then I have a master workflow with four command tasks calling the four workflows.
How do I isolate the connections so that each command connects to the target SQL Server database, so that each runs on a different processor?
Informatica also has functionality called "Partitioning", which does exactly what I did here. It splits the data up however you want. But there is some Informatica server level setup for this to work.