"Too big" to be a good breeding animal is not at all the same as being just "too big." There are MANY larger Havanese that are on the large side and are WONDERFUL pets. 10 lbs at 4 months definitely sounds on the larger side. Many Havanese are 10 lbs at their adult weight. The size of the parents is not necessarily a sure indicator of the size a pup will attain. My Kodi's partents are 11 and 12 lbs, and he his 16.5. I knew he would be big when I bought him, but didn't care. I was buying him as a performance animal, not as breeding stock.
That all said, if this is your first experience with Havanese, and you bought a dog sight unseen, you are probably not ready to start a breeding program yet anyway. Were your pup's parents both champions? Were they both completely health tested (with good results!)? Are you willing to show your dog to his championship, and neuter him if he proves not to be championship quality? Are you willing to do all appropriate health testing, including waiting until he is at least 2 years old to do final hip x-rays? Do you have someone who can mentor you in finding a bitch who is an appropriate mate for THIS dog? Are you willing to purchase her and go though all the same steps to prove that she is a good breeding candidate?
As you can see, there is a LOT that goes into RESPONSIBLE dog breeding. The fact is, that no responsible breeder will sell puppies with full registration and breeding rights to an inexperienced person. Part of being a responsible breeder is making sure that only the BEST possible puppies are allowed to continue in your lines. There are many dogs that have the conformation and temperament to make wonderful show/breeding dogs that are sold with limited registration to pet homes, just because that is the BEST kind of home for them!
A responsible breeder has a contract that both they and the buyer sign, that outlines all the details of the sale, including type of registration, when the pup must neutered/spayed (assuming limited registration), arrangements to take the pup back should the buyer need to give it up for some reason, health guaranty, purchase price, etc. If you have a contract, you need to read it carefully. If you didn't sign a contract like this, I wonder about your breeder. But it also makes me SURE that you are not ready to start breeding dogs either.
Please enjoy your wonderful puppy and get him neutered at the appropriate age. Of course you don't HAVE to register him, but it is inexpensive, and there are benefits, including a service to help find him if he is ever lost. You are also then eligible to compete in a number of fun and interesting performance sports, like Obedience, Rally and Agility.
If you find, after spending more time with your little guy, that you are TRULY interested in breeding Havanese, join your local breed organization. Go to lots of shows and learn as much as you can about top Havanese conformation. Learn about showing and breeding. Then find a good, reputable breeder who is willing to mentor you. Have them help you decide on breeding stock. Then show those dogs and have them fully health tested. You will still treasure your first "pet quality" boy because being a "pet" is the best thing a Havanese can do!