Is it ok to cite someone who provided a bad citation for correct information?
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.