I have a large string, I want to find 4 sequence Numbers in string. How do I get it?

For example:

```
string a=@"I(1946) am a string and I have some character and number and some sign like 4412 but I must find 4 sequence numbers in this."

```

result expected: '1946' and '4412'

```
string a=@"2015/10/13 is my birthday.I love this date1234!"

```

result expected: '2015' and '1234'

```
string a=@"xx888xx88x9xx99x9999xx"

```

result expected: '9999'