OP didn’t state where they are from, but there are plenty of places where it is extremely hard to fire, even if an employee is consistently underperforming.
Not saying that this is the case in this circumstance (I don’t know OP,) but the “you must be good because you lasted 4 years” heuristic doesn’t work that well outside of the US, or similar markets with at-will employment.
if they were sole dev in a startup for four years and couldn't ship software the startup probably wouldn't have lasted the last four years... Or they very quickly wouldn't be the sole full stack dev any more and relegated to some shit job
Even in France which lots of people call "socialist", the company can fire you at any time and has to pay only about 3/6 months of salary for "wrongful termination". That's low enough to not keep constantly underperforming people.
They could, but usually you don't want this to happen as an employee. Alternatively the court can force the employer to pay a severance if they regard the working conditions as untenable. An obvious example would be bullying at the workplace. But all of this is only applicable in the case of an unlawful termination.
Mostly if they want to get rid of you they offer you a severance.
It is also much more complicated than I can explain in a single post. For example, you can be fired for working "slowly", but you can't be fired if you are doing your best - even if all your colleagues are faster workers. It mainly depends on if you are doing your best.
E.g. your colleagues are able to dish out 100 pizzas in a typical evening but you only manage to make 50, and your boss wants to fire you. You don't agree and sue him. The court now has to decide if you perform so badly because of something that is "inherent" - which is called a "personenbedingte Kündigung" or in English dismissal on grounds of personal capability, or if it is based on your conduct. A dismissal on grounds of personal capability is usually deemed unlawful, as long as you give your best, if you consistently only manage to make 50 pizzas it seems like it, right? However if you willfully (!) perform badly the dismissal is usually deemed lawful. But, if you e.g. suffer from rheumatism and you can't perform the way you used to but you want to perform better, you just can't anymore it gets a bit complicated: the employer has to make a prognosis on how and if you are able to someday perform better again or if you could do another job. So, usually it is cheaper to offer someone a severance in that case.
Only if the courts decide that it was unlawful, but companies have enough experience and lawyers who make sure they have enough dirt on you to justify them firing you. It's what my German ex-boss told us and I've had seen many colleagues get fired without any repercussions for the company. Plus, if you end up sueing your former employer, and the word gets out, no other employer will touch you.
Also, for your mental health, you wouldn't want to go back and work at the company who just fired you. That's like getting back together with an ex you just got out of toxic relationship.
Big companies have lawyers up the wazoo, but startups (esp. with a single dev like OP) struggle with funds in many cases, so often hire lawyers to the minimal extent possible.
Not saying that this is the case in this circumstance (I don’t know OP,) but the “you must be good because you lasted 4 years” heuristic doesn’t work that well outside of the US, or similar markets with at-will employment.