And that's the problem. The 413 give that Beggar's Bazooka attribute "Hold Fire to load up to three rockets\nRelease Fire to unleash the barrage" but wait. I'm not done. With this attribute I managed to do it and it has nothing to do with #441 attribute (attribute that works like expected with Grenade Launcher but not with Rocket Launcher)
Made a little mixture here
Attributes
#6 - +<inverted_percentage>% faster firing speed. I set it to 0 for instant launch of all 4 rockets (should be set to 3 rockets though. I know how to make it. No worries)
#411 - +<additive> degrees random projectile deviation. Or else all 4 rockets would go through stright line which would make it look like one rocket.
And #413 in order to not be able to stop the barage. Also - it is mentioned that Giant Black Box Soldier does not fire until fully loaded (
http://wiki.teamfortress.com/wiki/Soldier_Robot) so in fact that Black Box might work in a manner Bazooka does. Although it possible for player to partially load Rocket Launcher (or Beggar's Bazooka in this case) before releasing rockets but let say bots were programmed in manner they MUST load every rocket before launching.
So yes. I guess the case is closed. Maybe someone else will get to work attrib #441 with maybe with other attrib if it is even possible. This would only make it more comfortable to use, that's all. Not a big deal really. Anyway, thanks for the tip Wliu!