$hour = date("H") & ~3;
$base = strtotime("$hour:00:00");
$list = array();

for($h=0; $h<24; ) {
    do {
        $time = strtotime("-$h hours", $base);
        $h += 4;
    } while(date("H", $time) == 4);
    $list[] = $time;
}

foreach($list as $time)
    printf(date("Y-m-d H:in", $time));

2016-02-06 16:00:00
2016-02-06 12:00:00
2016-02-06 08:00:00
2016-02-06 00:00:00
2016-02-05 20:00:00

var list = [];
var date = new Date();

date.setHours(date.getHours() & ~3);
date.setMinutes(0);
date.setSeconds(0);

for(var h=0; h<6; h++) {
    if(date.getHours() !== 4)
        list.push(Math.floor(date.getTime()/1000));
    date.setHours(date.getHours() - 4);
}

console.log(list);