This should work for you:

cat * | perl -ne 'BEGIN{print ">concat\n"}next if $_=~/^>/; chomp; print $_;' > concat.fa

This line concatenates all files in your directory and passes the output to perl. Then perl prints a header line (>concat) and every line (which does not start with >) without the newline.

Please try it on a test dataset and let me know, if it works for you ;-)

awk approach to combine sequences:

awk '/^>/ {if (c++ == 0) {print;}next}/^$/ {next}{printf "%s", $0}END {print ""}' input.fa > output.fa

The fasta header of the output sequence will be the the header of the first sequence in your input file.

A Python solution:

concat = ''

j = 0
while j < 4: # That is, the number of files you have.
    for line in open('gene' + chr(j + ord('a')) + '.fas'):
        if '>' not in line:
            concat += line
    j += 1

print '>concat\n' + concat.replace('\n', '')

cat *fas | grep -v '>' | tr '\n' '' > out.fas