The 4 year cycle isn't exact though be last halving cycle, we have waited for about a year to see the price from skyrocketing and during the halving there wasn't a lot of movement in the market. It was during Tesla's bitcoin purchase and their acceptance of bitcoin as a payment for their cars that affected the prices.
no it was not
tesla bought btc in december2020, thus tesla did not affect the market in feb/march2021
however MEDIA talked about tesla in spring2021
the speculation was not due to tesla itself not elon.. but due to media speculation
however, speculation drama aside. what actually mostly occured to cause the main ATH event, is that most mining farms dont audit their mining costs vs coins created daily/monthly. instead they buy mining hardware pre-halving(preparing) where hardware has a 2 year life cycle. and they buy electric not monthly but in 24 month contracts.
they simply mine no matter what the market is doing.. if a asic uses 3kw per hour (52,560kw/2year) they will mine non stop all 2 years, as thats what they already paid for.. there is no benefit stopping if markets change because they have a set unit of kw to use in a set time. or they lose it.
and when the contracts/hardware lifecycle are nearing their end. then they audit their earnings of coin vs the upfront cost.. and decide on resisting to sell at X amount or not.
so its linked with the hashrate vs reward. which helps support a minimum "bottom" no one wants to sell below. which then aids speculation above it. when some countries with expensive mining realise they are mining at xmultiple of market. they then happily just buy coin if they cant continue mining in the next mining cycle(2 years)
the best example is the 2013 event. when asics started. those old GPU miners realised they were dying in the competition where the sats per kw became low sats for high kw. so they just bought btc on the market. those preparing asics done their end of life costs of GPU mining. and realised now needing to buy ASICS would be an extra cost so they refused to sell coin below X. meaning it caused the market surge
these financial auditing practices of deciding to not sell below X and those now prefering buying is usually a year after the halving