The event that happens every 4 years is called "Bitcoin Halving." This could be seen as the process of reducing the total mining reward by half, controlling the rate at which new Bitcoin are produced (mined), and at the same time controlling the number of Bitcoin in circulation.
The first halving happened in 2012, when the reward for miners was reduced from 50 Bitcoin as mining reward for each block to 25, which is a total reduction by half of the actual figure. This kind of reduction happens every 4 years; the last halving was around 2020, which brought down the block reward to 6 BTC+ per block (don't know the accurate figure), so next year 2024 is the expected next halving. This kind of event is usually among things that trigger a bull run.