Reverse the effect of endianness on a memory dump

I've got a series of pipe commands that gets me a memory dump represented as a string of hexadecimal. This memory dump includes some ascii text that I want to read. The endianness of the memory dump means that whenever I pipe it to xxd -p -r, I get groups of 4 characters that occur sequentially, but where the characters within each group occur in reverse order. I'm looking for `` that would undo that order. For example, `` would do:

$ echo "lleHoW o!dlr" | 
Hello World!

or

$ echo "43218765" | 
12345678