Congrats on the new arrival! How exciting. You never know, it could be twins! I also have 4 (girls) and am expecting #5! My pastor's wife told me her midwife told her that third and/or fifth babies tend to be 'unusual.' With my fifth pregnancy I can certainly vouch for that- I also thought I was having twins (was praying for it!) and had more/earlier symptoms as well. There were two sacks, but sadly only one baby developed. Maybe next time! My pregnancy really has been very different with this one, though. I had preterm labor with my first four and took medications to keep them in longer (they were still all born early and were all breech.) This one had NO preterm labor, is head down and just about OVER DUE! Oh- and it's a boy after 4 girls! LOL Very different experience...
As for things you can do to increase chance of twins:
My husband's father's family and my father's family both have twins. It's my understanding, though, that it needs to come from your mother's side. My mother's side has twins, but none of them have survived to term (I have lost two, one with my first pregnancy and one with my fifth.)
If you ovulate more then one egg at a time you can have twins. This is where fertility drugs can help. With this pregnancy I ovulated several days apart from each other, and perhaps the twins were conceived several days apart (but one didn't develop properly.) In the future, if you are 'trying' to get pregnant, chart your cycle, and after ovulating (and sex) continue charting and you may find yourself ovulate a second time very soon after the first. This has happened to me more then once. Have sex again and you MIGHT get twins!
Wishing you the best with all your littles.
S., SAHM to 4 girls ages 6 yo (1-23) 5 yo (2-22) 3 yo (5-4) 1 YO (3-30) and expecting our first boy any day!