As a step-dad, I can be pretty sure that your current partner has known this day will come, so it will not be a surprise to him when you mention it.
My eldest daughters' dad and my wife were divorced when they were six and four, he had little, and then no contact with them at all. The oldest remembers the bad times,. and wants nothing to do with him, having seen him once when she had her young daughter with her. They were cordial, but it's apparent that neither of them was interested in the other. Our middle daughther, being something of a drama queen, went through a period of 'finding her roots' which involved contacting him without our knowledge, not that ww would have wished to stop her contact.
So, as someone in your partner's position, you need to dicsuss this with him. It's no-one's 'fault' - it's simply circumstances. Rest assured that your daughter's good relationship with your partner will ensure that she knows who her dad is (as opposed to her father - there is a world of difference!) and will get you all through this.
Personally, I would try to head of contact at this age - but every case is different, and you need to find out the views of all parites before you move forward on this.
Your partner needs to know - you have to be a couple on this, as you are with everything else.
Keep us updated won't you?