if a<10,000,a*0.08,if a10001,a*0.06, if a20001,a*0.04
You almost have the formula correct as you presented the issue.

You must keep in mind that the IF/THEN/ELSE function will stop considering
conditions, when the first TRUE path has been determined.

I also use the = and <= conditional operative.

=if(A1=20,000,A1*0.04,(A1<=10000,A1*0.08, A1*0.06))
-OR-
=if(A1<=10,000,A1*0.08,(A1=20000,A1*0.04, A1*0.06))
However, I like the format of showing the percentage rather than the decimal
equivalent as follows;

=if(A1<=10,000,A1*8%,(A1=20000,A1*4%, A1*6%))