Can I plug 4 Ohm speakers to 16 Output directly?
It's generally not a good idea to load a 16 &Omega; audio output with 4 &Omega; speakers.  It may not blow up anything, but you are putting 4x the load on the output amplifier it was intended to handle.  At best you get lower output power due to the impedance mismatch.  The amp could also start clipping and adding various kinds of distortion, and the frequency response may also not be as intended.

If you want to listen via speakers from something only intended to drive a earphone, extra power has to come from somewhere.  That power would generally come from a audio amplifier intended to drive the speakers.  Normal "PC speakers" contain amplifiers (which is why they have their own power supply), so those would work.  They present a relatively light load (often 600 &Omega;) and are intended to be driven from "line out" outputs.  These are signal, not power.  The amplifier in the speakers supplies the power.  The output of your music player should be able to drive such powered speakers without issue.  In other words, think of the music player as having a "line out" output rather than a power output.