This is a very random thread but I'm genuinely curious to see if anyone else has this problem. For whatever reason, I constantly get the numbers 4 and 6 mixed up. It's been happening a lot lately and it's pretty annoying. I know what 6 and 4 look like, I know what they both mean. Cognitively it's not an issue but, for example, I just caught a Suicune a little while ago. I counted how many pokeballs it took and it took either 14 or 16. I counted but then immediately forgot which it was. It could have been 14, it could have been 16, it definitely wasn't any other number. I hope I'm explaining this in a way that makes sense.
I almost want to say I suffer a similar thing with 5 and 7, but that's only in the context of me writing and reading haikus, never apart from that. Can you think of a possible cause for your association of 4 and 6 similar to that? Really, I can't think of any effective way to correct that other than paying more attention to your use of numbers, and consciously, perhaps speaking-out-loud verbally correcting yourself each time you notice it happens. That's what helps me overcome my mistakes and missteps: very consciously addressing them.
I'm not really sure what it is that causes me to associate them. One thing it could be is that I have a form of synesthesia that causes me to see colors when I hear sounds. It works a little in reverse too but not as strongly. The word "four" sounds blue to me and "six" sounds green. Sometimes the colors sound pretty similar though, like six is sort of a bluish green, so maybe that's why? The thing is, other numbers sound similar too. "One" sounds dark red, "five" sounds like just a base red, and "seven" sounds sort of like an orangish red and I never get any of those mixed up. But as I was writing this out, I kind of thought that a lot of words and sounds tend to be between blue and green on the color spectrum. That's something I've noticed before. Maybe I'm thinking something that sounds blue while also thinking of the number six, for example, and because there's more blue than green I associate it with four instead? I'm still not entirely sure that's the case though, since it never happens with anything other than four and six, but it's the best explanation I can think of.