How to replace multiple blank lines with a specific non-blank line

That's the job for sed:

sed -e 'G;G;G;G;G' file

With awk:

nawk -vORS='\n\n\n\n\n\n' 1 file

Or shorter version:

awk 'ORS="\n\n\n\n\n\n"' file

or avoid setting ORS for each input line:

awk 'BEGIN{ORS="\n\n\n\n\n\n"};1' file

Here you go with awk and processing the file only once.

awk -F'|' 'NR==1{print;next} m && NF{print m}
    NF{l="\n"$0; $5=$4; m="\n"$0; c=0}; !NF{c++}
END{ print l; for (; i++<c;)print }' OFS='|' infile

Explanation:

Here we are skyping first line to being replace 5^th field's value with 4^th field's value, and just print it and do next.

... if it (current next line) was not empty line (at least contains one field NF), then take a backup of whole line with a \newline added l="\n"$0 first next set 5^th field's value with 4^th field's value $5=$4 and last set it to a variable m with a \newline added m="\n"$0;; There is a c variable as a counter flag and is used to determine the number of empty lines !NF{c++} if no line with at least one field seen; Otherwise c=0 will reset this counter.

Now we have modified line in m variable and m && NF{print m} will print it where in the next step awk runs and m has set and it's not on empty lines & NF (this is used to prevent duplication on printing when empty line).

At the end we are printing the untouched last line which we take backup every time before performing replacement END{ print l; ... and then number of empty lines which never seen a line with a field with looping for (; i++<c;)print }'.

That's much shorter if you don't need redundant empty lines.

awk -F'|' 'NR==1{print;next} m && NF{print m}
    NF{l=$0; $5=$4; m=$0} END{ print l}' OFS='|' infile