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As the heading asks?

In terms of automotive uses - dot4 handles higher temperatures (higher boiling point).

In terms of paint stripping ... Dot3 is cheaper than dot4. Both are about the same level of efficacy at paint stripping.

DISCLAIMER: ALWAYS WEAR SAFETY GLASSES AND GLOVES (AKA PPE). You don't want ANY of this stuff in your eyes. That said, you SHOULD do this for ANY paint stripping method - from dettol to acetone, ethanol or isopropanol.

I've had no plastic melting/dissolving using dot3 and it removed (with some scrubbing - do not put it into an ultrasonic bath as it CAN ignite because it IS flammable) acrylics and enamel paint without much in the way of effort.
I did have some older styrene plastics have some softening using dot4 so I stick to dot3 on plastics.
I mostly use it on METALS though, when I can't get acetone so rarely have any issues with it.

* My method was:
Pour fluid (dot3) into a sealable jar - sufficient to cover the models you wish to strip.
Allow to soak for 30 minutes.
Remove from jar and scrub with medium toothbrush for about 20-30 seconds of back and forth.
This removed the majority of the paint. Repeat procedure if needed. 2 soak/scrubs were all that I ever needed, and that was to remove approximately 8 layers of caked on paint - a mix of unthinned acrylics and enamels. It even took etching primer (for metals) off.

Brake fluid CAN remove automotive lacquer (fresh new car paint) if spilled on it.

It can be filtered and reused for paint stripping (paper napkin or a coffee filter in a funnel). the paint residue and filter can be tossed into the bin (depending on your local area and dried paint waste regs). It will lose efficacy though.

Disposal can present an issue since you can't just pour it down the sink.
Find a mechanic who will let you dispose of it in their drum (my local gets his picked up for disposal by a chemical waste company).