Suppose 4 days before the day after tomorrow was Thursday. What day of the week was yesterday?

Monday

Sunday!!

Friday.

To solve this problem, we need to follow a step-by-step approach.

Let's break down the information provided:
- "4 days before the day after tomorrow" means counting back 4 days from the day after tomorrow.
- If we pretend today is tomorrow, then the day before tomorrow is today. So, today is Sunday.
- Now, if we count back 4 days from Sunday, we get Thursday.

So, if the day after tomorrow is Thursday, today would be Sunday.

Now, to find out what day of the week yesterday was:
- Since today is Sunday, yesterday would be Saturday.

Therefore, yesterday was Saturday.