Japan does not have any rifles for BR4 in their tech tree.
I propose adding the Type Hei with a longer barrel and bipod. It should be given 10rnd detachable magazines.
The extra length from the barrel could justify a minor damage and accuracy increase compared to the other Type Hei rifles at lower BRs. This should make it a fine addition for Japan at BR4.
Considering that we already have two Type Heis with 10 round mag (both in BR3), this suggestion is just to weaken the uniqueness of those GO weapons.
And you need some actual improvements to keep a rifle BR4 instead of BR3, such as 15 round mag.
The SKT rifle with a carbine barrel deals 13.9 damage, which can be used as a reference. The regular version of the Type hei uses 6.5mm bullets, and the long barrel can increase its damage level by one, which buff it to 14.4. However, if it only deals 14.4 damage, it is just another SVT38, which is BR3. Not to mention svt38 has faster reloading speed.
My suggestion is to add this rifle to the Japanese BR3
Type 4 corresponds to the Johnson 1941, which can be paired with the Garand as it has an 8-round ammunition capacity