The way you wrote it [=a1/4*(G19)], g19 was being mulitiplied by 4 and a1
was being divided by the result.

You might want to look at this topic in Excel Help: About calculation
operators

Not true. A1 is divided by 4, then the result multiplied by G19.

WRT precedence, as it says in Help, multiplication and division have the same
priority and "if a formula contains both a multiplication and division
operator — Excel evaluates the operators from left to right".

He wrote =A1/4*(G19). That is the same as =A1/4*G19, which evaluates as I
described above. If that's what's wanted, no parentheses are required.

Parentheses are needed only if you want to do what you have described, but the
the formula for that would be =A1/(4*G19), with the 4 inside the parentheses.