Since Japan and USSR have no Ground attackers I think this will be the most unique and really best options for this nations for BR IV:
Japan:
B7A2 (Homare 23)
The premium version of B7A2 (Homare 23) with its 4 × 60 kg Navy Type 97 Number 6 ground bomb
2 × 250 kg Navy Type 98 Number 25 bomb in the middle supported with 2 × 20 mm cannons will make this one a formidable opponent.
Here a short introduction to the plane:
USSR:
TIS MA:
Its with very similar battle rating to B7A2 and very unique compared to the other USSR ground attackers this plane combines good fire power and good payload it has worse flying characteristics but its a flying tank. It has 2 × 250 kg FAB-250sv bomb in the middle and 8 × RBS-132 rockets the rockets are weak but the bombs really compensate for that it has 2 × 37 mm NS-37 cannon with 90 rounds and 2 × 20 mm ShVAK cannon with 240 its very similar to Whirlwind Mk I that’s in the game but do to its rockets its one BR higher.
Here a short video on the plane:
That’s all for now :3 
