What does Romans chapter 15 verse 4 mean?

Sometimes, just a different translation can help... along with reading the passage in context with the ones around it.

"May the God of Hope fill you with all joy and peace as you trust in Him, so that you may overflow with Hope BY THE POWER OF THE HOLY SPIRIT.

" I myself am convinced, my brothers, that you yourselves are full of goodness [Paul is speaking of the Holy Spirit in them], complete in knowledge and competent to instruct one another." (Rom.15:13-14 NIV)