The only way you are going to know for sure is a test when the baby is born, is your partner aware there are questions about who the father maybe? With the scans and measures taken, they can give you a rough idea and for many are quite accurate however I remember with my daughter the entire pregnancy they were telling me I was two weeks less pregnant than I actually was. I only had one partner and I knew for dam sure the date of conception - Sorry I was meaning regards their testing not your issue if that came over snappy. Yet they insisted nope the baby was 2 weeks younger than the dates I was giving them...oddly enough she arrived at 36 weeks and a pregnancy is 40 weeks so who was right me or the measurements, and her size was 7.3oz so not a tiny baby.
Do you have a regular 28 day cycle normally? If you don't work backwards 14 days from when you would have been due your period to find out when you were ovulating. It's only a 3 day gap between the two guys and it's (for me) just too hard to call as sperm can live 5 days inside, and as you have been with your partner 4 years without pregnancy occurring, personally I'm going to stick with taking a test once the baby arrives I just wouldn't like to guess. If medic Dan sees this topic I think he's quite knowledgeable in areas like this maybe he can help you a little more than I've been able to.
It's to close to tell.
First off, there are no tests that can pinpoint exactly when you conceived. Every fetus develops differently, just like children. Ultrasound dating is approximate. Even your due date can vary by several weeks. Fertilization does not occur immediately, it can be several days.
As Violet says, sperm can survive about 5 days. Either man could be the father.
Normally you would ovulate between days 11 and 16 of your cycle. You had sex on days 8, 9, 13, 14, and 17. The 17th is unlikely, not impossible. The other dates are all possibilities.
You will need a DNA test.