Going by the numbers in your question, we have a four million margin on a total number of votes cast of over 143 million. That's less than 2.8%. Looking at the historical popular vote margins on Wikipedia, I would say that any margin below 5% is standard, not a landslide. For example, Obama's 2008 presidential election win had a margin of 7.27%.
The second metric by which you might consider it a landslide is by how much they win in the electoral college. If and by what margin that happens is too early to call in this election. It could be a decisive victory by number of electors if Biden wins most of the states that haven't been called yet.
Whether it's a landslide depends on the context. Again, we can look at Wikipedia which also maintains a table on the electoral college margin. In the last ten presidential elections, there have been two (1980 & 1984) where the winner won over 90% of the electoral college by number of electors. That's a landslide. In 2008, Obama got 67.84%, that's over two-thirds, so you could say that's a landslide as well. In fact, of those last 10 elections, 6 elections were won with a 67%+ electoral college margin.
At the moment, the New York Times predicts Biden has 253 electors 'in the bag', and the remaining states are good for 71 electors. In his best case scenario he would win all of those, giving him 324 electors. That's only 324/538=60.22% of the electoral college. It's decisive, but not really a landslide considering the electoral college margins in (recent) history. And that's only in the best case scenario.