If you had super regular cycles on the dot every 28 days, one might guess that the second guy is the dad. Unfortunately, that's out. If you continued to have sex after the 7th, then there might be a way to rule out the May 4-7 time period in favor of sex that came a little later.
But if the only sex you have had in the past many months was May 4, 5, 6 and 7 (none earlier than May 4 and none later than May 7) and now you are pregnant, the only way to know who the dad might be is by doing a DNA test.
Regarding DNA testing for paternity, if you have the big bucks a pre-natal DNA test costs, use Ravgen or the DDC and test with both men. If you don't, or want to save all your money for the baby, do a DNA test once the baby comes (which costs about a tenth as much as one done when you are still pregnant) and again, test with both men. Use a legitimate lab where people go with their ID, don't use a drugstore test.
Write back if you did continue to have sex with one of the guys after May 7. We might be able to figure out if the first guy at least could be ruled out. (Unless, of course, the sex after May 7 was with the first guy.)