Final answer:
The detail that shows that Jack and Bob are experienced inventors is the description of their workshop and laboratory containing expensive and intricate pieces of machinery.
Step-by-step explanation:
The correct detail in the passage that shows that Jack and Bob are experienced inventors is (4) This conversation took place one afternoon in the latter part of July in the basement of a house in Skowhegan, Maine. The room was fitted up as a combined workshop and laboratory, and a single glance would indicate that the two boys were by no means novices, for it contained many expensive and intricate pieces of machinery.