$4800 Medical Bill sent 5 years after services rendered (California)

Hey folks, the short version is I received a $4800 medical bill in September for services rendered 4 years, 10 months ago. I’m wondering if I have recourse in California… this seems obscenely late and frankly inappropriate to send a bill for services 4 years later. For additional context, read below.

This came as quite a surprise given that for this particular service I had spent 19 months (May 2018 -> Dec 2019 very deliberately trying to get seen by an in-network specialist to avoid having to pay $1500 for my copay that was quoted to me by my insurer (to say nothing about $4800)… This turned into an insane ordeal with an extensive email chain that ultimately contributed to my employer group dropping this insurer years later, but back to the incident in question. I had to request a “single case agreement” due to the fact that none of the list of 20 or so in-network specialists provided by my insurer were available to see me (either they specialized in a sub-field that wasn’t appropriate, or they weren’t taking new patients, or they were retired). My Single Case Agreement was ultimately rejected due to my insurers claim that there were in fact specialists in my area… the very same specialists I had already called and documented the lack of availability of. The two examples they provided me were both pediatric specialists who obviously would not see me given I was in my late 20s at the time and not a child. After months of ridiculous back and forth which included 6 separate referrals from both in network and OON specialists, and multiple rejections due to alternative options that weren’t alternative options, my insurer finally had someone from their A&G team set up an appointment for me at my local Hospital. I finally got an internal referral. I attended, got treatment, and forgot about the ordeal.

Now I just received a bill from the hospital saying my insurer paid $15k, and I’m on the hook for $4800. My first question was, why did my old insurer pay 70% of this bill nearly 5 years after services were provided. I called my old insurer (who I am no longer a member with) and they have no record of paying it… they contact the provider and the provider provides the name of a legal firm, with no contact information, who apparently covered $15k of the cost. There’s no way I’m paying this, as this is a product of incompetency on my insurers part which cost me 19 months of my life and somewhere in the vicinity of $500k in lost earnings due to delayed treatment, and delayed billing from the hospital. My question is what is the proper approach to extricate myself from this bullshit.