What is a 4-mix engine? Do you mean a 4-cycle engine?
Check it carefully! see if it has a place to put oil into the engine block, such as a screw in/on cap, and a dip-stick. This usually denotes a 4-mix/cycle engine.
A 2-mix engine would say to me that you should mix a certain amount of oil with the gasoline, ala 2 cycle engine. A 2 cycle engine doesn't usually have a dip-stick!
The terms you have stated don't mean much to us older mechanics. Is that something you have heard from the place where you bought it, or from somebody who is new to the trade? Better check with the seller, before you do something wrong, and end up with a useless piece of equipment that said:"BANG"! right after you tried to start it! Haste makes waste!

Paul,

The BR430, is NOT a 4-mix engine nor is a BR420 either.

Both of these are a pure 2-stroke engine just like the BR420. The only difference is the engines...the BR350/430 models have a stratified charged motor (it is essentially a large version of the BR200/BG86 engine) vs a single on the 420.

The only blowers with the 4-mix are the BR550,500 and 600

Rusty,

A 4-mix engine is a STIHL patented 4 cycle engine (has valves,cams, lobes that 2 cycle's don't have) but uses 2 cycle gas for lubrication purposes. Those engines do NOT have oil sumps.

They are essentially a hybrid 2/4 cycle motor, they've been around for at least 10 years or so.

If you familiar with STIHL's FS90, 110, 130 trimmers and BR500,550 and 600 backpack leaf blowers...those are your 4-mix motors...they sound different than regular 2-cycles.