John 3:18. Since no one is born believing in the name of Jesus, that's a potential problem for the age-of-accountability position. However, if your son's four-year-old believes in Jesus enough to have someone check on his home after any rapture then I submit he has more faith than many much older Christians and, since he's four, it's possibly not his little flesh that is driving that inquiry. It could be a fleshly fear of abandonment or a spiritual longing for his heavenly Father
(or something else).
The honest, forthcoming answer is
no one knows. what happens to the fetus, the newborn, the cognitively impaired, or the very young. Maybe, just maybe, God has seen fit, by grace, to make some unstated accommodation for those not sufficiently cognizant of the meanings of words
(although I am not sure why that would hinder the almighty Holy Spirit of God) and simply not included it in His revelation but maybe not. The fact is the Bible is predominantly concerned with the faculties of adults. It is, therefore a bit of a red herring to even ask about young children.