Now, to give you a bit of background: the author of Hebrews (whomever he or she may be, though my old-school grammar will use he all the way through here) has been comparing the Exodus of the Israelites to what God has done for all of humanity through Christ. The author is also contrasting our responses to the response of the Israelites.
What chapter 4 covers is that the Israelites were not able to enter the Promised Land because they lacked obedience. If you're familiar with the Old Testament, you know that God brought the Israelites to the edge of the Promised Land, told them to check it out and go take it. They checked it out, then decided it was too much trouble. Essentially, they said "We'd rather go die in the desert." So they did.
Something to notice about the Israelites then, and it's what the author of Hebrews is pointing out: they lacked faith, and so they failed to act. It's not the failure of their actions that leads to the criticism, rather it's the failure of their faith, but the failing of faith leads to disobedience.
So, what about us? We're not leaving Egypt, at least most of us. Rather, we're trying to walk in obedience in the life we have now. Guess what? Our faith is shown by our obedience. It's not shown by calmly sitting back and claiming we have faith. It's shown by our obedience.
This is what the author is saying: the word of God is united in our hearts only when we obey Him in faith. Have you partnered your faith with obedience? Obedience to profess faith, to live faith, and to share the faith?
And our faith, well, it's strengthened by our obedience. It's a self-reinforcing cycle.