awk(1posix) -F '\t' ' {sum += $4} END {print sum}' A.txt
pattern scanning and processing language
-F  ERE
       Define  the  input  field separator to be the extended regular expression ERE, before any input is
       read; see Regular Expressions .
program
       If no -f option is specified, the first operand to awk shall be the text of the awk  program.  The
       application shall supply the program operand as a single argument to awk. If the text does not end
       in a <newline>, awk shall interpret the text as if it did.

argument
       Either of the following two types of argument can be intermixed:

file
       A pathname of a file that contains the input to be read, which  is  matched  against  the  set  of
       patterns  in  the  program.  If  no file operands are specified, or if a file operand is '-' , the
       standard input shall be used.

assignment
       An operand that begins with an underscore or alphabetic character from the portable character  set
       (see  the  table  in  the  Base  Definitions volume of IEEE Std 1003.1-2001, Section 6.1, Portable
       Character Set), followed by a sequence of underscores, digits, and alphabetics from  the  portable
       character  set,  followed  by the '=' character, shall specify a variable assignment rather than a
       pathname. The characters before the '=' represent the name of an awk variable; if that name is  an
       awk  reserved  word  (see  Grammar ) the behavior is undefined. The characters following the equal
       sign shall be interpreted as if they appeared in the  awk  program  preceded  and  followed  by  a
       double-quote ( ' )' character, as a STRING token (see Grammar ), except that if the last character
       is an unescaped backslash, it shall be interpreted as a literal backslash rather than as the first
       character  of  the  sequence  "\"" . The variable shall be assigned the value of that STRING token
       and, if appropriate, shall be considered a numeric string (see Expressions in awk ), the  variable
       shall  also be assigned its numeric value. Each such variable assignment shall occur just prior to
       the processing of the following file, if any. Thus, an assignment before the first  file  argument
       shall  be  executed  after  the  BEGIN  actions  (if any), while an assignment after the last file
       argument shall occur before the END actions (if any). If there are no file arguments,  assignments
       shall be executed before processing the standard input.
source manpages: awk