You need to work on your titles, if not else 
(your title makes it sound like you think this is caused by the jack and not the amplifier/speaker combination)
Driving a 4 ohm speaker with line out doesn’t really work; you need an amplifier designed for low-impedance output, not a line out buffer designed for high-impedance inputs. The LM386 amplifier is designed for 4-32 ohm speakers, but the volume you get out depends on how it’s configured, supply voltages, and also the speaker efficiency. Have you tested the amplifier with some other source?
You may also want to check the existing output threads, btw; see this topic and the topics linking to it: