Two things I noticed right off, one is you use $2-5 instead of $2-4 which would allow extra data in since you never check if (!$5).
The second thing is..

if ($len($2) == 4) && ($2 isnum) && ($len($3) == 4) && ($3 isnum) && ($len($4) == 4) && ($4 isnum) {

Could be changed to

if ($len($remove($2-4,$chr(32))) == 12) && ($remove($2-4,$chr(32)) isnum) {

I know its not a lot shorter but it does give you a great example of creative thinking to bring down the amount of ifs you need.

You should also consider checking the input against the file to make sure it doesnt already exist to avoid someone adding the same number a thousand times.

The code in general is a very simple one, almost too much so.

Keep up the coding. :)