Well it depends if you count 1 as a Prime number. Most people don't and so the forth would be: 2, 3, 5,... 7. If you do count 1 as a prime number then it would be 5.
A prime number is divisible only by itself and by 1. Wikipedia lists the first 25.
http://en.wikipedia.org/wiki/Prime_number
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It is: 7
5 x 7 x 4 = 140
its a composite number because 75 can be 5 x 15 = 75 so on and so forth
"Forth" is not a number.
7 is a prime number!