We cannot by all our sufferings, any more than by our services, merit heaven; but by our patience under sufferings, we are prepared for the promised joy.
The main message of 1 Thessalonians 4 is to live a holy life, free from sin and worldliness, and to love one another. To be sanctified, which means to avoid sexual immorality and to live a sexually pure life.
Paul is facing the worst of all hardships: his own impending death. So, he encourages Timothy to stand strong in his faith, with a reliance on the written Word of God.
For bodily exercise profiteth little: but godliness is profitable unto all things, having promise of the life that now is, and of that which is to come.