What are the chances of Biden actually lasting the four years?

Literally 50/50, just like anyone else, he could drop dead tomorrow, or years from now.

Much better than Trump living 4 more years.

Start with Trump's 12 Diet Cokes a day, then add his vegetable free diet and love for junk food. At 100 pounds over his proper weight, clinically OBESE Trump is bound to have another heart attack, the two in office didn't kill him, but they say 3's a charm, so start planning his funeral now.  

 Plus I doubt Trump will survive facing all the criminal charges against him. 

If the stress doesn't kill him, watching police arrest his Jewish daughter, her Botox husband, and his two idiot sons,  Eric and Don JR. should at least cause him to have a stroke. 

No one at Mara Lago will give Trump CPR, if he croaks there. Because there's no telling what porn star he's had his mouth on. I bet Melania and Barron would go shopping if Trump needed an ambulance, leaving him, rather than helping him isn't going to change Trump's will.  

There's a great chance! Maybe even eight. Si se puede!