Both R406A and GHG-X4 (now called "Autofrost", "Chill-it," and other names) are 100 percent legal for use in cars.

As far as using a modern R134a refrigerant (like in a 1995 Neon) with a new dryer, compressor, hoses, etc. here is my experience: You're looking at replacing some components either way, but the switch to R406A is going to require MUCH less expense, MANY fewer new parts, and will yield MUCH better performance and efficiency than R134a. Basically, DO NOT(!) convert your R12 car to R134a or any of the R134a-based refrigerants (FRIGC comes to mind). They are far less efficient than R12 and R406A, GHG-X4, etc. Also, they are HIGHLY incompatible with R12-type oil. They operate at radically different pressures, so calibration of R12 expansion valves won't be optimal. Also, since 134a is so inefficient, one must use larger compressors, condensers, and evaporators to get the same level of cooling as from an R12 system. This is difficult and expensive to do on an existing system, so you get much less cooling with 134a. To convert to 134a PROPERLY so as not to shell any of the components, you're going to need:

a) All new hoses (The R12 hoses are permeable to the smaller R134a molecule)

b) All new seals and O-rings (ditto above)

c) A rebuild of your V2/RV2 compressor (because this compressor design uses an oil sump and pump which cannot be "flushed" of oil like other non-sump type designs, and because 134a is so violently incompatible with 12-type oil, you have to get ALL of the old oil out.)

d) New receiver-dryer with XH7 or XH9 desiccant

e) And you should really purchase a parallel-flow condenser to try and make the system work at least marginally, if not optimally well.

Now let's compare that to using R406a or GHG-X4:

They are MORE efficient than 12, yet it operates at similar pressures, so the expansion valve calibration will remain optimal and your existing condenser and evaporator will function MORE than adequately.

They are completely compatible with R12 type oil, so no system flush or expensive compressor rebuild.

You still need new hoses, because R12 type hoses are permeable to the smaller R406a factions' molecules.

Ditto new seals. (New hose-to-hose and component-to-component seals, you don't have to mess with any compressor or other internal seals).

The choice is crystal clear here, isn't it? Now I have to ask YOU a question: Why are you converting to anything? R12 is still around, you know, and anyone who tells you otherwise is lying or ill-informed. There is plenty of it in the US. There is MORE than plenty of it in the US. Some garages will tell you that R134a is the only legal refrigerant. That's wrong. Some will tell you that 134a "moves more heat" than R12. That's also wrong. Some will tell you that no flush, rebuild, or anything else is needed when converting to 134a. That's dangerously wrong. Either use 12, or use 406a, or use GHG-X4.