I’d like to know why the forecast temperature on Windy changes so much for Ezeiza Airport (SAEZ).

15 hours before, it showed a high of 16°C.
8 hours before, it showed a high of 15°C.
4 hours before, it showed a high of 17°C.

It is not surprising that the forecast max temperature for a given location varies by 1°C or 2°C each time the model is updated. After all, why else would updates be necessary every 4 hours? A single update per day, or even per week, would be enough.
With each update, the model recalculates the forecast based on thousands of observations that have, of course, changed over the course of those 4 hours. The 1°C or 2°C variations you mention with each model update are observed worldwide, not only on this airport.

the forecasts are often inaccurate, especially when I compare all four major weather models.

The forecasts are not inaccurate. Each model makes its own calculation which gives some differences. This is the reason why Windy offers several weather models.
To me, the key question is determining which model is the most accurate for a specific location. The "Observation vs. Forecast" feature allows you to compare model forecasts with actual readings from a weather station.