Next on the receiver types is the 4B, which is made for the underfolding AKMS rifles. This AK has the Type 4B receiver.
Picture courtesy avtomatsinaction.
The Type 4C receiver was then added and it incorporated an accessory mount on the left side of the receiver to attach optics. Here is a picture of an AK with the mount…
Picture courtesy avtomatsinaction.
…and with the mount and a sight mounted.
The last of the Type 4 receivers was the Type 4D, which now incorporated a latch mechanism in the front left side of the receiver to secure either a solid or skeleton folding stock. Here is a Type 4C/D receiver with both the accessory mount and the latch mechanism.
Picture courtesy avtomatsinaction.