How many points for all four safeties?
Assuming you're playing an edition by Parker Brothers (where the game is played over multiple hands and the first to 5,000 points wins), you get a hundred-point bonus for each safety played, and another 300 bonus if you play all four safeties, according to https://gamerules.com/rules/mille-bornes-card-game/ and https://www.hasbro.com/common/instruct/MilleBornes.pdf :


  SCORING 
  
  Score as many points as miles traveled ... X
  
  Bonus for 1000 mile trip ............... 400
  
  Bonus for safety card .................. 100
  
  All 4 Safety Cards played by 1 team .... 300
  
  Each Coup Fourré ....................... 300
  
  Delayed Action ..........................300
  
  Safe Trip .............................. 300
  
  Shut Out ............................... 500


The 300 points bonus for al four is explicitly "in addition" to the 100 points for each one:

(Again from https://www.hasbro.com/common/instruct/MilleBornes.pdf ):


  All four Safety Cards played by the same team (add this bonus to the 100 points scored for each safety card): 300


And also confirmed on Wikipedia


  All 4 safeties: 300 bonus in addition to the standard 100 points for each of the safeties individually plus any coup-fourré bonuses.


ADDENDUM:

If you're playing the Dujardin edition, the games is just one hand (ending at 1,000 miles), and there's no bonus for playing all four safeties.