It's true that over astronomical timescales, the future of our solar system will bring significant changes. The Sun is currently in the middle of its life cycle as a main-sequence star. In about 5 billion years, it will exhaust the hydrogen in its core and enter the red giant phase, during which it will expand significantly.
Estimates about how far the Sun will expand during this phase vary, but some models suggest that it may indeed be large enough to engulf the inner planets, including Mercury, Venus, and possibly Earth. However, this event is not expected to happen for several billion years, not just a few million.
The mention of 3 to 4 million years might be a misunderstanding or mixing up shorter-term solar activities with the long-term evolution of the Sun. The processes that lead to the Sun's eventual expansion are much longer than that timeframe.
If you're interested in discussing more about the future of our solar system, stellar evolution, or any related topics, feel free to ask!