I have tried several variations with 'sed'

use Perl :)

#!/usr/bin/perl -p

s/(\d+\.\d+\.\d+\.)(\d+)/$1${\($2-1)}/;
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Usage:

$ ./decrement.pl infile >outfile
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Update: alternatively, you could use the /e option, which treats the substitution part as Perl code (instead of as a double-quoted string):

s/(\d+\.\d+\.\d+\.)(\d+)/$1.($2-1)/e;

# or (just to show another variant)
s/(\d+\.\d+\.\d+\.)(\d+)/"$1".($2-1)/e;
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