Most cash surprises don't come out of nowhere. They show up because nobody was looking a few weeks ahead — payroll is due Friday, a vendor wants payment now, and the bank balance is the first place anyone finds out there's a problem.
Why 4 weeks, specifically
A full 13-week rolling forecast is the right tool for ongoing cash management, but it's more setup than most people want for a first check. Four weeks is short enough that your assumptions about what's coming in and going out are still grounded in things you actually know right now — invoices already sent, bills already scheduled — rather than guesses.
The one simplification worth knowing about
A quick 4-week check is usually built on one simplifying assumption: that money lands in your account the same week you expect it. In reality, that's rarely exactly true — you invoice this week, get paid next week, or the week after. For a fast gut-check, that simplification is a reasonable trade-off. If the number that comes back is close to your safety threshold, that's exactly the moment to move to a more detailed forecast that models real payment timing instead of assuming instant payment.
What to actually do with the answer
If every week comes back comfortably above your threshold, you've confirmed what you probably suspected — good, now you can stop thinking about it for a couple weeks. If one week dips close to the line, that's specific and actionable: you now know which week, which gives you time to move a payment, follow up on an invoice, or delay a non-essential purchase before it becomes urgent.