He wouldn't be given all the subtests not because he couldn't handle them but because they don't have norm data for this age group. In order to get a score they need to compare your child results to the results of the same age kids.
Not surprisingly comprehension reading wasn't administered to the sample of 4.5 year old kids they use as a norm. Therefore they don't have any numbers to compare your son's scores to and he cannot obtain any meaningful score. You can still obtain grade and age equivalent and perhaps scaled score compared to 5 year old or the first month of K but you won't get 4.5 year old score. Most testers won't even administer such subtest.
DS4 can read a chapter book cover to cover in one sitting all by himself but unfortunately that won't show up on achievement testing till he starts K.
