Above are two screen grabs from the RealClearPolitics.com polling of Hillary Clinton versus Donald Trump (left) and Joe Biden versus Trump (right) in comparable periods of time September 4, 2015 to April 18, 2016 and September 4, 2019 to April 18, 2020.
According to RCP's aggregate of polls, Biden leads Trump nationally by 5.8 points, 48.3 to 42.5 as of today. Four years ago Hillary Clinton led Trump by 8.5 points, 49 to 40.5.
There was more fluidity in the polls four years ago, with Trump closing the gap a couple of times in a similar window of time. Of course, Trump came close enough to surpassing Clinton in nation-wide polling on election day that he was able to pull off an electoral win. Since the beginning of the year, Biden's lead has been close to five points and has varied very little.
Biden's current lead over Trump is smaller than Hillary Clinton's lead four years ago. However Biden has maintained a much more steady lead over Trump., which indicates that voters are much more settled in their opinions of Trump and Biden.
As alluded to above, US elections are not a national popular vote contest. It is a series of 50 state (plus the District of Columbia) contests which will result in an Electoral College win for either Biden or Trump.