Don’t be surprised if Apple replaces your Apple Watch Series 3 with a Series 4 when you turn it in for repairs. The company informed their staff via an internal memo that they may have to substitute Apple Watch 3’s due for repair with the newer Series 4 due to low inventory.
This means that if you have a GPS + Cellular 3 Series Watch and visit an Apple Store to have it repaired or replaced, they may fulfil it with a GPS + Cellular 4 Series Watch due to constraints.
For the moment this applies to Apple Authorized Service Providers and Stores all around the world, but there’s no additional information on whether the fulfilment upgrade is temporary.
The same thing has happened before- in January, Apple began offering the Series 2 for Apple Watch users who went in for 1st gen and 42mm Series 1 repairs when the stock started running low. During the first week of May, a similar Apple watch substitution was temporarily put in place where Series 2 repairs were to be fulfilled with Series 3 models due to a parts shortage.