Error in Number sequence

I am created Number sequence for one field 0001,0002 … like this

but error is when i opening the form every time number sequence is changed

example :

before closing the form Number is 0002,

next time i am opening its changing 0002 to 0003;

again close the form again next time i am opening its changing 0003 to 0004;

004 to 005…

if i open form number of times then numberseq is change to new number … for same record

like this

i wrote the code in data source , active method

I got it