I saw the term 〇〇〇〇おじさん written in a chat profile. What does the 〇〇〇〇 mean?
〇〇〇〇 implies that various words are applied to 〇〇〇〇. For example, he might say 映画好きのおじさん、公務員のおじさん、車が趣味のおじさん and so on. But he doesn't say it concretely because he would just tell that he is a middle aged man or the word is one that he doesn't want people to know such as 変態, ドスケベ.
〇〇 is used to mask/censor part of a word.
In contexts related to otaku-ish hobbies, Xおじさん is recently used to form a slang word that means "(stereotypical) middle-aged male X fan". The nuance is more or less similar to 大きなお友達. For example ガルパンおじさん (fans of Girls und Panzer) and アイカツおじさん (fans of Aikatsu!) are the two best-known examples of this, but there are other minor 〇〇〇〇おじさん's. Since there are exactly four circles and there is no particle between
おじさん, I feel he may be referring to either of these two works. Or else, he may be just implying he has some otaku-ish hobby.
It probably means that the person does not want to reveal his [sur]name, but wants people to know that he is male and that he is not a teenager, but rather in his [I would say] 40s or 50s.