The MS4B is of the Catkin construction, with a copper anode sealed directly to a glass envelope and then enamelled.
The MS4B was made in both catkin and conventional glass envelope versions, but the electrode system designed for the glass envelope version would not fit the catkin envelope. Thus the /K version was redesigned and became superior to the glass version.
It was designed as a amplifier tetrode for RF and IF applications in broadcast radio receivers.
The MS4B (introduced 1931) was an improved version of the MS4 (notably, higher gm). The /K version is a classic Series 1 catkin dating from 1933-4. Series 2 catkins (1934) were taller, with B7 base caps and tapered external shields.
Note the dual-purpose top cap connector suggesting that this specimen is of relatively late manufacture. The Type VMS4/K (also Series 1) were externally identical to the MS4B/K, except for marking. Both types also exist without external shields but are rare in this form.
The distinctive Osram Catkin label is not often still in good condition.
The screen over the copper-glass envelope is 33 mm in diameter and, excluding the B5 base pins, is 109 mm tall.
Here is an example without the external shield. The glass to copper join, the copper anode and the top-cap fitting are all clearly visible.
Close-up the join between the glass and copper can be seen on the rear right of the picture. The screen grid is an enclosing wire mesh.
References: Datasheet, 1005, 1040 & 1043. Type MS4B was first introduced in 1931.