The basis for that is the Soviets used other Howitzers to range the targets, then altered the calculations for the B-4, which was then fired and was as a result extremely accurate.
While historically using lighter Howitzers to barrage served to disguise the upcoming attack by zeroing in for the B-4, in-game it would serve as an alert that a B-4 is about to fire on that area instead.
Oh so thats how they nailed the Finnish bunkers down during winter war, basically they somehow managed to land direct hits to the bunkers, collapsing them and killing everyone inside (Kinda off-topic but using other howitzers to gain an accurate shot on enemy positions was totally new information for me, so thanks!)