Thank you....I tried your first solution but if we have a digit on the fourth line the program add it also. for example: 
@NS
ATGGCTG
+
@#FBH66     2

[download]
the output comes as 
@NS
ATGGCTG
+
@#FBH66     68

[download]
how do I rectify it. and how to check the memory consumption of the script.

In reply to Re^2: merge multiple files giving out of memory error by Anonymous Monk
in thread merge multiple files giving out of memory error by Anonymous Monk

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