How to pronounce "$ a:b=c:d$" in English?

"$a$ is to $b$ as $c$ is to $d$."

Or,

"The ratio of $a$ to $b$ is equal to the ratio of $c$ to $d$."

On a rather pedantic note, this isn't an issue of "pronunciation" per se, but one of "verbalization." How do we speak or verbalize a mathematical construct (in this case, an equation between proportions).

In English, we pronounce $a:b :: c:d$ as "a is to b as b is to c", where the colon in particular is pronounced "is to".
The $::$ sign that I have used in place of the equality sign is the proportionality sign.
So if you wish to say $a:b = c:d$, then you will say "a is to b is equal to b is to c".