I've been using tank top propane heaters in my greenhouses for years with good results...
These heaters are the best source of emergency heat in my opinion:
http://www.google.com/search?um=1&h...e+heater+&aq=f&aqi=&q=tank top propane heater
I use the single burner version, usually 40 bucks or maybe half that on sale at the end of the season.
For 400 sq.ft. you would probably want more than one burner - separate burners each on their own tank will give more flexibility plus the tank will last a lot longer with only one burner on it.
I used to keep two 15,000 BTU heaters in my 200 sq.ft. greenhouse, and currently use one heater in each of my 120 sq.ft. greenhouses.
I only run the heaters when temps are forecast below freezing, and mostly use the Low setting (9000 BTU) except for really hard freezes.
On the low setting one 5 gal tank will last about four nights (48 hours), though different brands of heaters will vary a bit on their fuel consumption.
I highly recommend using an oscillating fan in the heated area to mix the air and get a much better distribution of warm air throughout the heated area, assuming the area is enclosed.
Besides heating the air, these heaters also produce infra-red heat...
These heaters will produce a lot of water vapor which actually helps to keep spider mites down in the winter months.
I've never had any ill effects on the plants from the heaters, though this year will be my first time over-wintering crotons in the greenhouse - been all cacti and succulents up til now...
