In Report No. 3, we showed that there was no remarkable difference between the reaction stress in way of master butt, which concerns with residual stress and intensity of restraint, and reaction stress in common butt, from the actual ship experiment on oil tanker “Andros Tanker”, 41, 000 D. W. This result seems to be unnatural, because it is considered that the condition of restraint to common block butt and master butt are obviously different. To solve this problem, further experiment was carried out on the other ship of 73, 000 D. W. oil tanker. First, the mechanical force was applied to upper deck block twice, firstly as erected and lastly before welding, in order to observe the block behaviour. This experiment was conducted to find the fundamental relationship between the displacement of block and the external force. Next, we compared thus obtained actual result with a presumed curve which can be drawn by considering the ideal case in which there exist no other than block friction. Through the above experiment, in actual ship, it is found as follows : (1) The outer force which is sufficient to keep the newly erected block moving increases slowly until the block butts contact, and after rapidly. It should be pointed out, however, that the displacement of block is taken place by considerably smaller force than that expected in consideration of elastic deformation of block. It is considered that this fact is due to the dislocation or destruction of root face at the block butt. (2) Comparing the displacement of block at common block butt with at master butt, rigid block motion takes place at the former, while it does not at the latter. (3) The difference of reaction stress between common block butt and master butt is less than 2 kg/mm² which agrees with the result in Report No. 3. It follows from the above that it is not the rigid body motion of block that govern the residual stress, but the degree of the self-restraint of weld in connection with thermal effected zone. (4) Thus, it is concluded that master butt under the Method of Erection Started Simultaneously at Several Locations, as is used in this ship yard may be treated quite in the same way as in the case of common block butt, provided that master butt weld properly carried out, for example adopting the method of simultaneous weld by many welders.