Order by rand() in yii2

Wrap it into yii\db\Expression to prevent escaping and remove array part:

use yii\db\Expression;

...

$query = User::find()
    ->where(['category_id' => 5])
    ->orderBy(new Expression('rand()'))
    ->limit(4);

I'm not really an expert on yii2 query builder, however, specify the ordering should help

$data= User::find()->where(['category_id'=> 5])->orderBy(['rand()' => SORT_DESC])->limit(4);