Connecting 240v 14-50R to multiple heaters in parallel
This is not legal or safe, but it's safe"er" at minimum cost. 

Get a 120mm (4-11/16) junction box and lid, a cable clamp appropriate for that oven cord, 4 cable clamps appropriate for the heater power cables, and three rather large wire nuts, or alternately, 5/6-position lug splices that'll take up to #6.  Also 1 foot of bare 10AWG wire, and a #10-32 ground screw (style points for green). 


The 120mm box has a 10-32 tapped hole in it, put the ground screw in there and attach the pigtail. 
Knock out holes for all the cable clamps that will make sense when it's mounted to the wall, and fit all the cable clamps, screws out so you can get to 'em. 
Permanently mount the box on the wall within cord's reach of the 14-50 receptacle.  Certainly do not leave it dangling loose. You'll pull cords out that way and give yourself a wallop. 
Bring the dryer cord into its cable clamp. Tighten it down. 
Bring each heater cord into a cable clamp.  Tighten it down. 
Use a splice to tie all the following wires: a) the stove cord ground, b) the pigtail from the junction box, and c) 4 ground wires from heater cords.  There better be ground wires in those heater cords! 
Use a splice to tie all the following: a) the heavy black wire in the stove cord, and b) one of the conductors from each heater cable (doesn't matter which). 
Use a splice to tie all the following: a) the heavy white wire and b) the remaining conductor in each heater cable.  Yes, you heard right. White. 
Put a wire-nut cap on the red wire, and tape it on so it doesn't fall off. Wire-nuts don't like to stay on single wires. 
Put the cover on the box. 
Plug in the 14-50 plug.  Light this baby up!  


If you are getting the expected/advertised heat, then you are wrong about the heaters being 240V.  They are 120V. By hooking up this way, we just saved the heaters. 

If they're all heating but the heat is 1/4 of what you are expecting, then you passed the test.  Unplug the 14-50 cord.   Reopen the box,  and redo the last 4 steps, but attaching the red wire to the heaters, and capping the white.  Now you should get the works.

This isn't legal because these are not permissible uses for flexible cords, and we're abusing the tap rules, and these heaters are not listed to be portable or temporary heaters.  The proper way would be a subpanel, 20A breakers feeding 6-20 receptacles, and proper cordage to heaters that are made to be portable.