The top left corner is (r0,c0) so we have to check from row r0 to r0+2 and from column c0 to c0+5 checking that every square is free.
break allows us to leave a for loop early.

function hor_clash_check(r0,c0) {
  var ok=true;
  for (r=r0;r<(r0+3);r++) {
    for (c=c0;c<(c0+6);c++) {
      var a=get_a(r,c);
      if (a!=7) {ok=false; break;}
    }
    if (!ok) break;
  }
  return ok;
}
function hor4() {
  for (var k=0;k<500;k++) {
    var r0=Random0(7);
    var c0=Random0(4);
    var ok=hor_clash_check(r0,c0);
    if (ok) {
      set_a(r0+1,c0+1,3);
      set_a(r0+1,c0+2,2);
      set_a(r0+1,c0+3,2);
      set_a(r0+1,c0+4,4);
      break;
    }
  }
  return ok;
}