‘Because’ is the most direct and complete word for giving the reason of a thing; for example: I could not arrive on time because the flight was delayed by three hours [here because suggests a direct cause]
‘Since’ is less direct and often indicates a succession in the chain of reasoning, or a natural inference or result; for example: since all men are rich, some have to be rich [here we have a kind of inference derived from the fact that all men are rich] you must remember that since is weaker than because.
‘As’ is weaker than ‘since’, and might be used in cases where one is expected to be courteous; for example: as he was not well, I thought of paying him a visit [here replacing ‘as’ with ‘because’ will bring out the awkwardness of ‘because’; ‘since’ can still be used, but would sound less polite]
‘For’ shows a mere suggestion; there is no cause-effect connection whatsoever, and is therefore the weakest of all the four; for example: in the bible we have ‘’Father, forgive them, for they do not know what they are doing.” [Here ‘for’ is a mere suggestion; it does not suggest any cause effect relationship; as, please get up my friends, for the sun has risen]
