Thanks folks. I ended up getting a few more 4' flouro's and its been keeping the temp nicely. Almost too nice because I also grow hydro fodder for the chickens in this shed and mold becomes an issue at temps above 70.
I think if I were to insulate the bottom of the shed with some sort of skirting or even better some 3" insulation panels, I could probably stay above 60deg F with outside temps in the teens.

david, what scenario are or were you using that heater? We actually have some winter weather up here so I don't know that it would be sufficient.