4800 watts? Is it a big greenhouse, or lacking insulation?
One or two small fans should provide enough circulation to break up cold spots, most often low, along the base of a wall, or in corners. I'd mount them high, pointed down, to circulate the rising warm air above the electric heater.
Here's a thread on a functional cold-climate greenhouse, built in 2009.
The thread "Low-tech Solar Radiant Heat" in
Greenhouses forum
