Richard's answer is right on. However, reading your question, I wonder if there is a maybe a miscommunication or misunderstanding? They didn't mean it would actually take 4 months of active work to upgrade you, did they? Wouldn't that mean you would be down for 4 months?
They most likely meant that between planning/testing/new hardware/etc., the whole process would take 4 months from when you give them the go ahead to completion. Depending on the size and complexities of your GP environment that might be entirely reasonable.
Or maybe they meant they are booked for the next 4 months and can schedule your upgrade in 4 months? Without more context and a lot more detail, it's impossible to really answer what you're asking.
That said, most of the upgrades that we do are completed over a weekend. You stop work on the old system on Friday. On Monday you have the new system available. However, we have done upgrades where the planning/testing/etc. took several months because of:
- waiting for new hardware/software
- waiting for approvals
- availability of internal resources
- issues discovered during testing
- waiting for 3rd party vendors and support
- other projects scheduled