Since I have a primary home and investment home A, I am having trouble financing investment property B through a traditional mortgage. 

Before your tenants move in, be sure to address any pre-existing issues in the property to avoid future headaches.

i'd do a lease addendum adding the new roomie, for the 3 existing tenants to sign, and have the new tenant also sign a new lease packet. you want to be sure they're on board with all the terms of the lease that the others agreed to. :)

We are Spearhead Mortgage and contact on my profile or on website. 

For example, Fannie Mae loans if an existing first mortgage is being paid off through the transaction, it must be at least 12 months old at the time of refinance, as measured by the note date of the existing loan to the note date of the new loan.  

I was able to move an existing tenant from an older less expensive property into this one and increase rent.

However, we are wondering if the interest rate will likely be higher for a cash out refinance on a property we have already lived in for a year and are about to start renting than the interest rate would have been if we just got a mortgage when we first bought the property.

I'm a local investor, real estate broker, mortgage broker, hard money lender.

This really depends on if you are using a mortgage to finance the property.

How can I maintain my current mortgage on the property?