I asked my preferred lender this question on Monday, actually, and she explained that it is based on the social security number. It has nothing to do with your marital status (or lack thereof). Each of you would need to qualify on your own for the mortgage to have 4 each. If you had a mortgage jointly, that would count as one for each of you. So moving forward, you could get a mortgage on your own (up to 4), with your partner (up to 4 each), or your partner could get separate mortgages (up to 4).
Remember that the limit of 4 DOES include any primary residence. They don't distinguish between a home you live in and an investment property. Four is four.
I hope this helps you. As with any mortgage question, it is important to talk to a lender you trust before making any decisions. I have several I work with who understand the laws and keep abreast of the changes, if you need a referral. Feel free to contact me.