How do I insert a space every four characters in a long line?

Use sed as follows:

$ echo "foobarbazblargblurg" | sed 's/.\{4\}/& /g'
foob arba zbla rgbl urg

You can use the following simple example:

$ echo "foobarbazblargblurg" | fold -w4 | paste -sd' ' -
foob arba zbla rgbl

Here is the example using grep and xargs:

$ echo "foobarbazblargblurg" | grep -o .... | xargs
foob arba zbla rgbl