The yield a staker receives from the protocol is composed of 2 parts *:
- Issuance
- Tips
The issuance is defined programmatically by the protocol and is dependent on the number of validators (stakers). There is a formula for it that basically scales with the square root of total amount of ETH staked. This is the part that gives ~4% nowadays. Given a number of stakers this part is guaranteed yield as it comes from new issuance. It's still subject to market dynamics in that if there are very few stakers, the yield is more interesting. If there are a lot of stakers the yield is lower.
The tips are completely market dependent and are a function of network activity and the price of ETH. When you pay a transaction you can include a tip for your transaction to be included faster. Low levels of activity lead to lower tips, high levels of activity result in higher tips. Tips are basically denominated in USD but paid in ETH, so high price of ETH leads to lower yield from tips, low price of ETH leads to higher price of ETH from tips. This part of the yield is obviously completely dynamic and subject to market forces.
With current (last 3 months) levels of activity the yield a validator can expect to experience is around ~5.5%. If network activity were to go back to Q4 2021 the yield would raise to ~25% at current prices.
*Technical note: There is a third part which is an effective yield resulting from burnt ETH and even a forth part that is outside of the protocol called MEV.