I have 2 kids, and have also had a very early miscarriage. But don't let the fear of miscarriage ruin your happiness. There is nothing you can do about it, try not to worry, it isn't good for the baby. :-)
also, the cramping is normal, you have 3 other kids, so keep in mind that your body is more sensitive after having more than 1, you'll feel things a lot more, plus, remember your uterus is getting bigger, and with you having had 3 already, you'll show a lot sooner, so that means that your uterus is stretching sooner, and that will cause cramping and uncomfortableness.
Congrats! and enjoy your happiness. Try not to worry until there is something to worry about (I know easier said than done)
Best of luck!
D.