R134a is a low pressure refrigerant, very efficient for high temperature applications.

This makes it ideal in air conditioning applications that may leak refrigerant like car air conditioning.

Now it is being phased out because of its high global warming potential and replaced by R134

R404a is a high pressure refrigerant ideal for applications in low and medium temperature.

You want low pressure refrigerants in household applications like refrigerators that may be inside kitchens with (possible) high ambient temperature, so you make sure nothing will blow up. For the same reason you want R134a in your car air conditioning.

R404a instead has pressures slightly above atmospheric in low temperature applications where R134a would have negative pressures and its heat transmission coefficients would be much lower.