My highlander has hfc134 refrigerant in it. Can I top it off with 134a?
Just different names for the same thing. So, yes.
Do not use one of those so-called "recharge cans" to do that.
R-134a is a HydroFluorCarbon (HFC), HFC-134 is just another name for the chemical and describes its makeup. The old R-12, the original Freon is a CFC (ChloroFluorCarbon) and was also called CFC-12.