Firstly, some perspective: IQ scores are not considered stable until about age 8 or 9, so barely non-qualifying scores at age 4.11 don't necessarily mean your son is not gifted.
Secondly: Yes, that is a wide range of subtest scores on both clusters (8 scaled scores difference), which usually is an indication that the cluster/IQ scores are questionable. Notably, if the examiner had calculated the cluster scores using the five-factor model, Visual Spatial would have come out as a relative strength, probably in the upper 90s %ile, likely above 130. (I don't have my manual with me, so I can't check these numbers exactly--plus if you're not in the USA, your norms will differ slightly.) Knowledge would have come out as a relative weakness, around the 50th %ile. The Working Memory Cluster has a notable divergence between nonverbal and verbal tasks, and not in the direction I would have expected based on the relative strength in Visual Spatial. It may be that the passion for numbers that you mentioned in your other post is related with his strong verbal working memory performance (repeating random number sequences).
Thirdly: Knowledge is most closely associated with what is called crystallized intelligence, which is what you have already learned from your environment (as opposed to fluid intelligence, which is how you adapt to new learning, and make connections between existing learning). Obviously, it is closely associated with schooling, and access to knowledge in the form of books, life experiences, and shared cultural experiences. It is not unusual for this area to be lowered in individuals who come from a minority cultural/linguistic/religious experience, or have poor access to literature/television/museums/parks, or are very sheltered. Or, obviously, who have not received any formal schooling.
You say your son is reading at about a late first grade level (short chapter books, Frog and Toad, Amelia Bedelia). How long has he been reading? A lot of general knowledge comes from books, so if he hasn't been reading independently all that long, he hasn't had time to accumulate certain kinds of knowledge. The Verbal Knowledge subtest, in particular, is a straight vocabulary test, so if he hasn't had exposure to the reading vocabulary yet, the lower score is no surprise. Give him a year of reading, and his score on this type of task would likely go up by more than a year's relative growth. Nonverbal Knowledge is an absurdities task, which is highly culturally-laden.
I would follow your son's lead in terms of feeding his thirst for knowledge, without worrying too much about classification for the moment. Wait another year, or until there is a specific purpose to be served by re-testing, and then have him re-examined using a Wechsler (if you wait until after 6.0, he can have the WISC-V, or if before, the WPPSI-IV). He can't have the SB for at least another two years, as IQ tests are not supposed to be re-administered within two years.
