I need a way to get the next available number on the sequence.

I have a list of number 

0001

0002

0004

In this this i should identify the next available serial should be 0003 then following 0004

Try something like:

While ( [
listOfValues = YourSequenceHere ;
result = ""
] ;
IsEmpty ( result ) and ValueCount ( listOfValues ) > 1 ; 
[
v1 = GetValue ( listOfValues ; 1 ) ; 
v2 = GetValue ( listOfValues ; 2 ) ; 
expect = SerialIncrement ( v1 ; 1 ) ;
result = If ( v2 ≠ expect ; expect ) ;
listOfValues = RightValues ( listOfValues ; ValueCount ( listOfValues )  - 1 )
] ;
result 
)

This will loop through your sequence and stop when it finds a break (not necessarily a gap) and output the next expected value at the break point. If no break point is found, the result will be empty.

I am not sure why you need this. If this is intended to generate a serial number for records there's probably a better method.