To be more precise, Trump was worried about mail voting and he promised to challenge the results if there would be reason to suspect fraud. That is not an outrageous position to have.
And there were indeed suspicious circumstances: an unusual amount of voters in 100-120 age group, ballots arriving late for Biden in critical states, strange count increases during the night, even software glitches in Biden's favour. But as I've mentioned 4 million lead would be hard to explain by simple fraud. It would have been a massive coordinated underground operation. And I'm not a fan of conspiracy theories
EDIT: my point is that you should upgrade your mail voting system that such incidents would be impossible to occur.