If the VFD's can accept an analog input, use a 4-20 mA output or 1-5 V transducer for the temperature based speed control.
If the units can accept a Master / Slave relationship or a follower input, set one unit as the master and slave the other three off of it.
Larry C
Thats right. Contact your favorite VFD manufacturer and check with them. The VFD is no longer just a speed controller, I have seen them with PLC's on board (ABB)and will accept every imaginable input and can output too, on some models. They can work alone or with other VFD's and equipment as needed.
In fact, contact ABB through their web site. Then locate a local vendor. They are great about touting their products and will answer any questions you might have.
Note: Do not confuse ABB with AB!!!! Totally different.