Is 4 a prime number?
4 is not a prime number, because it is divisible by 2: 4 = 2^2, and it has 3 factors.
Checking 4 for primality, divisor by divisor
| Try | Into | Gives | Verdict |
|---|---|---|---|
| 2 | 4 | 2 remainder 0 | divides exactly |
You only need to test primes up to the square root of 4, about 2: if 4 had a factor bigger than that, its partner would be smaller and would already have been found.
The quick checks first
- Divisible by 2? Yes: the last digit is even.
- Divisible by 3? No: the digit sum is 4.
- Divisible by 5? No: the last digit is 4.
- Divisible by 9? No: the digit sum is 4.
- Divisible by 10? No: the last digit is 4.
All the factors of 4
1, 2, 4 (3 factors)
Prime factorisation: 22.
The primes near 4
The prime before 4 is 3; the prime after it is 5.
🧮 Calculator
The answer appears here, with the working in words.
✏️ Five questions, answers hidden
1. Is 5 prime?
Answer: yes
2. Is 6 prime?
Answer: no
3. What is the smallest prime factor of 8?
Answer: 2
4. List the primes between 10 and 30.
Answer: 11, 13, 17, 19, 23, 29
5. Up to which number do you need to test divisors for 13?
Answer: the square root, about 3
📚 The class this belongs to
Prime and composite numbers are taught in Grade 4 and 5 in the US, Year 5 in the UK (KS2), Class 5 and 6 in CBSE and Year 5 and 6 in Australia.
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