The etiquette on Steemit is very much defined, and enforced, by the users with the largest investments in the platform. They have the most to lose if Steemit fails, and the most to gain if it does well. Only very recently, the account Hive account@grumpycat started to flag posts older than 3.5 days that bought upvotes after this time.

But I've just put $0.5 SBD into minnowbooster on a post that's 4 days old.

I would not worry. The upvote buying that is being countered is on the order of hundreds of SBD, and flags are only used to return huge paid upvotes to zero. You should not even crop up on the radar.

The best time to buy upvotes is soon after posting. This way, the extra vote weight helps your post get onto the Hot and Trending feeds which gives you exposure. After a post is several days old, even a massive upvote is unlikely to return it to those feeds.