possibly... but in a proper setting and atmosphere, it would not be a problem. is your battery compartment ventilated? read along...
when batteries are charging up.... or when they discharge at a very high rate, they give off hydrogen gas.... sometimes quite a lot under heavy charging conditions.
VERY explosive.
the batteries being in an enclosed area and possibly without ventilation, you could have a small tank of hydrogen waiting for ignition.... from ANY source.
so with all this said, it may not be the heater that is the problem, but the lack of ventilation in the battery compartment.
with that many batteries there needs to be some way to ventilate the space... hydrogen is lighter than air, so it wants to rise.
there are others who will chime in and give some ideas on how they have the ventilation set up in their battery area....
now that I have spread all the doom and gloom, with the heater being 4ft away, unless the gas escapes near the heater, it will probably rise away from the heater or be so dissipated by the time a whiff of it got there, that it would never ignite...
if you arent charging, and havent in awhile and arent under heavy usage loads, then I would not worry about it myself.... just keep the battery area closed off from the heater area.
but make sure you either have a vent for the battery compartment, or get one installed, because the ignition source can come from anywhere, such as a wiring issue below the liner, or switch being thrown when the gas concentration is just right...