There's a sequence of what's easier and harder in an instruction.
Passive phrases are harder to understand than active ones, but for that instruction, I would have at a minimum expected a 4 year old to at least get a book from downstairs (maybe not the one that was next to the pencil case). It's a command containing two tasks (go downstairs and get the book), but with a passive phrase modifying which book. (Edit: see below, it appears this spatial sense is not yet in place for a 4 year old. Learned something new...)
I love the PBS development tracker, because it's sufficiently detailed, and it really helps sort out normal bounds on average development, while making it clear what's out of bounds of normal development. This is the site that convinced me to get a speech eval for my then 3.5 year old son, and this is the site that later helped reassure me that DS' math development was highly unusual.
See the second example under "Language Comprehension" here:
http://www.pbs.org/parents/childdevelopmenttracker/four/language%20.html
