Not sure how they are creating "backblast" exactly. But I'm sure there may be a more simple method rather than creating a second short range invisible projectile that comes out the back of the AT-4 and explodes.
How is it possible? Well, I may suggest to see if the explosion effect of a simple claymore anti-personnel mine will provide the close range cone shaped rear explosion effect we're looking for. So basically have the AT-4 fire it's projectile as it normally would, but also simultaneously have an "invisible claymore" attached to the rear end of the AT-4 and have it detonate at the same time the AT-4 launches it's rocket. Bam! Then there's your "backblast". All that needs to be done is to have it's effects tweaked and refined.
However, creating the "backblast" effect still may produce a problem with players firing the AT-4 from a prone position. As basically the lower half of thier body is exposed to the backblast effect... Of course, it's not exactly ever a good idea to fire rocket launchers from the prone position, unless specifically done properly.
Heres some information in relation to the proper stances while utilizing AT-4:
Firing Positions for the M136 AT-4.
Heck... If "backblast" is finally implemented into the game you may even want to commit these facts to memory if you're not already roughly familiar with them in some manner.
Also, thanks Ninja2dan for the info. I myself am a large fan of the military and have spent a signficant amount of time becoming familiar with weapons from a civilian level as much as possible. I thought about joining the military a few times in my life, but that never came to be. I appreciate you sharing your accurate knowledge on these topics. Not sure if you're in the military, but it's not impossible to be well informed about military procedures from a civilian level either.