4 years is more than enough time for heat damage to occur, it's not just how often a month you straightened but also how long. Eventually the heat damage will catch up to you. I have seen people only straighten for two weeks, or even just once and get heat damage. Not everybody can straighten without consequence, you are drastically changing your texture by burning it.
Heat cuases the hair strands cuticle to refuse to lie down eventually and once the cuticle (outer layer of hair strand) does not lay down most of the protection of your hair is gone. The cuticle is what keep moisture in your hair once conditioned (ie flat) so your hair will be harder to moisturize becuase moisture can escape hair strands. The now protuding cuticle layers will catch more easily on each other, cuasing more tangles. They will also make the hair feel rough to the touch, than if they were flat (more strawlike than smooth/silky). Hair will also be more frizzy becuase frizz happens when hair seeks moisture from the air, meaning dry hair, meaning hair where cuticles are not conditioned. I'm not exactly clear on why hair loses elasticity but it does, which in curls means hair can no longer bounce back to it's curl pattern (shrinky hair is healthy hair) meaning looser curls or even completely straight areas. Hair will also break easier and be duller in appearance (shiny hair comes in part from a sleek cuticle layer becuase light bounces off of it better).
If your hair has all those symptoms it's probably heat damage.
There is no cure, anything you do from this point on is damage control and you are only covering it up. If you want your hair TO LOOK like healthy hair and generally approximate the FEEL of healthy hair (basically a disguise) some people have experience complete or at least some reversion in appearance and feel from protein and deep treatments. Aphogee 2 step is one protein treatment. Also Aubrey Organics GPB. Do several of these and the damage should be mostly covered up, though with the amount of time you straightened it's doubtful your hair will completely bounce back.
If you want your hair to ACTUALLY BE healthy that is a longer, harder process. Protein and deep treatments are also a part of this process but you recognize them as the coverup and not the cure that they are. Keep doing these, they only temporarily help hair by keeping cracked cuticle layer together. You have to do them often again and again. So while you are doing damage control you have a choice. You can cut off most or all your hair immediately and let healthy hair grow in it's place. Or choose option 2, which is let healthy hair grow in which holding onto damaged hair. Chances are you will see a texture difference, since healthy hair will still be the same tight healthy hair as before and at the demarcation line the damaged hair will be a much looser, tangly texture. The line will be tangly itself. Keep both textures, until you are ready to cut off damaged hair.