in verse 3 the NOT is the one used normally for requests and commands
so this verse should be taken, not as a statement which verse 4 will virtually repeat, but as a wish or prayer, to be answered with the ringing confidence of verse 4 and all that follows.
I.e., "May He not let your foot be moved, may He . . . not slumber!"--followed by the answer,
"LOOK, He who keeps Israel neither slumbers nor sleeps."
