When you have a verb from the fourth stem (باب الإفعال), its subject (فاعل) causes its object (مفعول به) to perform the action indicated by the same root letters in the ground stem (الثلاثي المجرد). Therefore, it is used to make a verb transitive (متعدي) if it is intransitive (لازم) in the ground stem. E.g.,
جرى - He ran.
أجراه - He made him run.
جلس - He sat.
أجلسه - He sat him.
نَجا - He escaped.
أنجاه - He set him at liberty./He saved him./He rescued him.
If the verb is already متعدي in الثلاثي المجرد, it is used to increase the number of مفعول بهs by one.
For example, the following verbs already took a مفعول به in الثلاثي المجرد:
أكل الخبزَ - He ate the bread.
آكله الخبزَ - He fed him bread.
رأى الشيءَ - He saw the thing.
أراه الشيءَ - He showed him the thing.
The following verbs already took two مفعول بهs in الثلاثي المجرد, so in باب الإفعال they take three:
علِمتُ زيدًا فاضلاً - I knew that Zaid is excellent.
أعلمت عمرًا زيدًا فاضلاً - I told Amr that Zaid is excellent.
If the ground stem has an adjectival meaning, i.e. 'to be(come) X' (where X is an adjective), then the fourth stem means 'to declare (something) to be X' or 'to think (something) X/to find (something) to be X'; e.g.,
بخُل - He was (or became) miserly.
أبخله - He thought him miserly./He found him to be miserly.
جبُن - He was (or became) cowardly.
أجبنه - He thought him cowardly./He found him to be cowardly.