Would topping of A Dot 3 only brake system with synthetic Dot 4 brake fluid cause the break pedal to get soft ?

I have a 2009 Chrysler Sebring 2.4 litter. Last night I topped off the brake system with synthetic Dot 4 brake fluid and now the pedal is soft and goes all way down to the floor but the car does stop when pressed. Not sure if it was an existing issue since this is not my daily driver or mixing fluids could have caused it. Any help is greatly appreciated! The brake reservoir does state to use Dot 3 brake fluid only.

Thanks,

 

A soft brake pedal going to the floor is not because of the DOT4 you added.
Sounds to me like air got into the system, and it needs to be bled.
Since the system will need to be bled of air anyway, just refill it with fresh DOT3.
DOT3 and DOT4 fluids are very similar. I believe the major difference between the two is the DOT4 has a higher temperature rating.

 

The brake pedal will go all the way to the floor normally if the engine is running. Try it with the engine off. Pump it down a few times to exhaust the vacuum in the power brake booster. Then press on it. If it still goes all the way to the floor, then you'll need to bleed the brakes properly. Antilock brakes may need a special procedure.