The number „4” („sa” in Korean) is a number which is rarely used and avoided as it’s connoted to the word death (“samang”). In terms of NK leadership, I do believe that Jang Sung Thaek was never the number 2 of North Korea . Instead of it, I do believe that Jang was only responsible for a lot of investments within the DPRK. His management policy and his fractionprobably disturbed Kim Jong Eun and therefore led to his execution.
I also strongly believe that a lot of Jang’s friends are being targeted and they will probably decide to defect in order to escape execution. Some of these people are living not far away from us: some are localized in Europe (maybe in your country ?)
The number 2 can be seen in the management of the Organization and Guidance Department of the CC KWP. A description is available there.