Light aircraft pilot reported an NMAC with another light aircraft in the vicinity of SABLO intersection.

Narrative

While on an IFR flight plan in VMC conditions; I was given direct SABLO intersection at 4;000 MSL. Approximately 5-10 miles from the SABLO intersection I was advised of converging traffic off to my left that 'appeared to be in a descent'. When I spotted the traffic; I estimated the aircraft to be between 4;000 and 4;300 feet. ATC had told me that I was cleared to climb to 5;000 if needed. Initially; I did not want to climb because the aircraft appeared to be slightly above me; however after observing the aircraft's flight path I initiated a steep climb to 5;000 feet. Around 4;400 feet MSL; the converging aircraft passed from left to right; just ahead and under me; missing us by what I would estimate to be between 300 and 500 feet. Based on ATC's callouts; I do not believe this aircraft was on flight following of any kind at the time. Incidents like these are why I believe it's imperative to fly at the correct VFR cruising altitude. Although I do not know the exact altitude of the other aircraft; to me it seemed that the other pilot was cruising at an IFR cruising altitude without talking to anyone; creating a collision hazard. Had this aircraft been on flight following; he may have been aware of my position; altitude; and direction; and would have also been able to take corrective action; further nullifying the situation. Resources such as flight following and ADSB-in can be extremely helpful in improving situational awareness; as this entire scenario took place in under 40 seconds.