Can I install FF4 Pro on two OS's?

Unfortunately not: You need several licenses if you want to install Filter Forge on multiple systems.
Here’s the quote from the Filter Forge EULA, the Grant of License section:

d). Standard License. Unless otherwise specifically provided under valid invoicing, purchase order or licensing terms accepted by Licensor in writing (e.g. trial, beta, or multiple user license as specified in a contemporaneous purchase order or agreement) granted by the Licensor upon registration of the Product (online or otherwise as provided by Licensor), Licensee is granted a non-exclusive and non-transferable license to Operate one (1) copy of the specified major version of the Product by one user account in the operating system (“User”) during the Term of this Agreement on one Client Device provided that if multiple or volume License is purchased, the number of the Client Devices and the number of permitted users shall be as provided and permitted by invoicing terms or applicable terms and conditions set forth on the Licensor’s website www.filterforge.com at the time of purchase of the License. The Licensor reserves all rights not expressly granted herein.

Hmm...technically though, multiple OS on a single machine, as Mar implied, couldn't be executed at the same time. So it shouldn't violate the EULA as only one (1) copy of the license is ever being used at any time, as well as they being installed on the same machine to boot, I think?

SpaceRay.
As I said answering your other post, when you use FF, just use it in only one computer in one time.
That's all.

Just think.
What's the reason for the rule?
The reason is to prevent that somebody gives his license to other people and that more than one person uses FF with only one personal license.
If you follow this, no company in the world would lose its time to give you troubles.