Paul seems to indicate he could have written at length about rejoicing in the Lord (Philippians 4:4) and God's peace (Philippians 4:7). These were certainly topics he would have enjoyed. Instead, he summarises a list of areas of importance for believers. They include what is true, lovely, just, commendable, pure, excellent, honorable, and praiseworthy.
Believers were to "think about these things." While God guards our hearts (Philippians 4:7), we are also commanded to focus our lives on things that please God. There is an ongoing back and forth throughout Paul's writings, which indicates God's involvement in every aspect of the life of the believer. At the same time, believers are commanded to live according to God's ways. He does the work, yet gives us work to do. Believers are called to trust in the Lord, yet also to serve the Lord. Paul set an example for how to do both. He was faithful in prayer, yet gave every bit of his life to serve the Lord.