Your question speaks to the very heart of the matter when discussing the size of one's family. I sense that you have found so much fulfillment in your role as mother and nurturer that you want to add to the joy you are experiencing. It is true that when we give of ourselves, instead of exhausting the resource of love in our hearts, we instead find that love growing. Love that grows longs to be shared.
What a wonderful witness that you and your husband are practicing Natural Family Planning. There is a great sorrow of our day and age. Too many times people make choices that are, for all purposes permanent, and then as their situations change or their spirit grows they want to take back what they have done. This is indeed the real sorrow. Because you and your husband are remaining open to the promptings of the Holy Spirit, then through prayer the Lord will show you the correct path to take. Pray to know His will for your family and He will honor that prayer.
I will tell you something that you may find hard to believe. When my husband and I were only married about 3 years (2 children) we had an opportunity to visit my cousin in California. At that time my cousin had 12 children.
My husband was overwhelmed by the sheer numbers of everything!! As we drove away, he told me quite seriously that he felt 3 children would be all that he could handle. I listened quietly. The years rolled away, as you can see the rest is history. But here is the interesting part. Despite my husband's reservations at the size of my cousin's family, he would feel strongly that each pregnancy was a gift. His strength and conviction was a wonderful encouragement to me. I had always wanted a large family. Looking back I now feel that though he may have had his doubts so many years ago, he was ready and open to accept the gifts the Lord might send. He just didn't know it at the time. :)
I suppose that I would encourage you and your husband to remain open to the Lord's call. Continue to pray together and ask for the guidance of the Holy Spirit. Speak candidly with your husband and discern his reservations. Listen with a heart that is ready to understand.
Jesus, we ask that You guide this dear family. Lord, they have found great joy in the children that You have blest them with to this point in their life. We ask that You continue to send the Holy Spirit to them each and every day as they search to know Your will in this life. Lord, give all parents and persons a heart that is open to life. Our reward will be a peace and joy that comes from You and Your grace. Amen.
Remember to praise the Lord and thank Him each evening during your family rosary for the gift of each child and each other. This is a strong and loving witness to your children.
Sending out a prayer,