に関して details

Examples

about N; regarding N — the subject matter under discussion, in a stiffer register than について

その件けんに関かんして、何なにか知しってる？

sono ken ni kanshite, nanika shitteru?

Do you know anything regarding that matter? casual

事故じこの原因げんいんに関かんして、現在げんざいも調査ちょうさが続つづいている。

jiko no gen'in ni kanshite, genzai mo chousa ga tsuzuite iru.

An investigation is still ongoing regarding the cause of the accident. neutral

本件ほんけんに関かんしまして、担当者たんとうしゃよりご連絡れんらく差さし上あげます。

honken ni kanshimashite, tantousha yori go renraku sashiagemasu.

Regarding this matter, our representative will contact you. formal

に関する is the adnominal form before a noun: 環境かんきょうに関する記事きじ "an article regarding the environment."

において
Both skew formal/written, but において marks the SETTING or domain something occurs or applies within (会議において決定された "was decided at the meeting"), while に関して marks the SUBJECT MATTER being discussed (会議に関して意見がある "have an opinion regarding the meeting") — the two overlap only when the setting and the topic happen to be the same noun.
について
Both mark the topic under discussion, but について is the everyday default in both speech and writing, while に関して skews formal/written even once the surrounding sentence turns casual — 週末について聞いてもいい？ reads unmarked, its に関して twin still reads a shade stiffer even in casual conversation.

に対して
Both take the shape に＋kanji＋して and both can loosely translate "regarding," but に関して names the SUBJECT MATTER under discussion (この問題に関して話し合う "discuss regarding this problem"), while に対して marks the target of an action or attitude and cannot introduce a mere topic of conversation the same way.