In a master's thesis the author provided a seemingly incorrect citation. ( Link to paper citation 4)
The information is obviously correct but in the cited paper I couldn't find any reference to the information he cited. Is it ok to cite him?
If you want to refer to his work (i.e. his results, discussion or conclusions), you should cite it.
If you want a reference to the particular piece of information he cited (the sentences describing plasma etching before his call to ref. 4), then you should find a direct source: either a textbook or review on the topic. This would be much better than a research article (or thesis), especially one badly sourced.