“When it fails” is a typical answer that will leave you in a tight spot and put you at risk of data loss and a crippling interruption of your business due to rushed decision making. Starting to considering a new server at 4 years is smart as a reasonable server life is 3-5 years. Please note a range like that depends on growth, usage and so many variables. Servers that retire later than 5 years however show an increase in mechanical failures like fans and hard drives. The costs of upgrading hard drives can exceed the costs of replacing the server and the interruption and risk are both higher. Keep in mind that a retired server is still running and can be turned back on and act as a backup to the installer! A good business model would start planning the server’s future retirement at around 4 years and choose their moment based on business flow over the next few quarters.
Consider you might have tied up your IT company with financial constraints that no one could be successful with. Over the years we have met many company’s shopping for IT as they tie their current company’s performance to an aged and failing server. This can be an oversight by the IT company in not having suggested the replacement in time or it can be unreasonable expectations on budget and results by the company constraining the IT company. We have been on both sides of that – just something to think about!