modifying WP Query

Hello,
I'm trying to modify the query bellow I tried to change the line $the_query = new WP_Query( array("post_type"=>"project_post") );

to  $the_query = new WP_Query( 'cat=4'  ); but it shows empty result on the page, my goal here is get all posts from specific category

original code:

<?php
                                                $the_query = new WP_Query( array("post_type"=>"project_post") );
                                                while ( $the_query->have_posts() ) : $the_query->the_post();
                                                $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'project_preview_big' );
                                                $image = $image[0];
                                                $categories = wp_get_post_terms($post->ID,"projects_category");
                                                $category = array();
                                                if(count($categories)){
                                                      foreach($categories as $categ){
                                                            array_push($category,$categ->name);
                                                      }
                                                      $category = implode(",",$category);
                                                }
                                                else{
                                                      $category = "";
                                                }
                                                $cats = array();
                                                foreach($categories as $cat){
                                                      array_push($cats,$cat->term_id);
                                                }
                                                $cats = implode(",",$cats);
                                                ?>