The 6b4 body armor is kind of controversial. Not as a protective, equipment but more about if it ever was present in Afghanistan. The short answer is - yes, it was present in Afghansitan. But you should not use it for reenactment. And the answer for this is longer and is explained in this article.
The adoption of 6b4
The 6b4 body armor vest was adopted at the same time as 6b3. While having a similar look (to a anyone who is not deeply invested in soviet body armors), these two were really different in many ways.
But first, let's talk about the similarities. Just like any previous (and some future) body armors, the internal part of 6b4 composed of a number of small titanium and boron carbide protection plates. The 6b4 had the biggest number of them out of all Soviet protection vests - 60 pieces, including one on each shoulder and four to protect pelvic area.
Apart from that, the pouches for equipment which were attached to the vest were similar to both 6b3 and 6b5. The 6b4 vest featured four magazine pouches, which could fit any of the Soviet AK or RPK magazines, it had four grenade pouches at the back, two utility pocuhes on each side and a traditional big pocket for tent (plash-palatka).
But the 6b4 body armor had its fare share of differences, compared to its predecessors and successors. Because it was so well packed with the hard plates, it did not have the soft yellow kevlar armor - just the plates. However, there was one massive improvement which balanced the chances. The 6b4 was the only Soviet Army body armor which had a damper to scatter the force from the incoming projectile. It was not unusual for the soldier to survive the hit only to get significant trauma from the hard armor plate.
All in all, this body armor turned out to be rather successful in terms of personal protection - it had the biggest protective surface and the dumpers to dissolve the hit. But this came at a cost - both for the user and for the industry. The 6b4 body armor turned out to be the most expensive vest out of all adopted in the Soviet Army - its was priced at almost two new Zaporozhets cars, just under 6000 roubles.
6b4 in Afghanistan
Dispite common misconception, the 6b4 body armor vest was, in fact, used in Afghanistan by the Soviet ground troops. The misconcestion comes from the fact that there are only a handful of documental historical photographs from the era, which show soldiers and officers wearing 6b4 vests.
This leads us into the situation, where it is unclear if 6b4 should ever be used for Soviet-Afghan War reenactment. Luckily, these vests are pretty rare and we will never see them being used in big numbers, simply because there aren't any for sale. So the situation does not change - 6b2 and 6b3 are still the only viable options for the reenactment of themed airsoft games.
In terms of its practical use, this particular vest was not well suited for dismounted operations in the mountained areas due to its weight. This might be one of the main reasons we see very little photos of the 6b4 being worn. Weighting between 13 and 17 kilograms (depending on modification) it was close to impossible to wear this body armor for long periods of time, while operating on foot.
The 6b4 body armor primary intended use for sentry duty, sapper work and close quarter firefights. In these cased, the weight would not matter too much and the person wearing the vest would have the maximum possible protection.