changing closing h4 tag to opening

Fixing what looks like a closing h4 tag where it should be an opening tag.  With that being inside two closing tags, "Internal definitions and block structure" looked like part of the normal paragraph text before it and read as an incomplete sentence.

Verified with another SICP text that it is in fact a subheading that is supposed to be there, tested this in a browser and looks good and "Internal definitions and block structure" shows up the side bar table of contents.