Charles H. Spurgeon on Psalms 139:4

For there is not a word in my tongue, etc. The words admit a double meaning. Accordingly some understand them to imply that God knows what we are about to say before the words are formed on our tongue; others, that though we speak not a word, and try by silence to conceal our secret intentions, we cannot elude his notice. Either rendering amounts to the same thing, and it is of no consequence which we adopt. The idea meant to be conveyed is, that while the tongue is the index of thought to man, being the great medium of communication, God, who knows the heart, is independent of words. And use is made of the demonstrative particle lo! to indicate emphatically that the innermost recesses of our spirit stand present to his view. —John Calvin.