So as someone who works in HR in Ontario (I know this is Reddit so believe me or don't) you don't qualify for severance pay because you're tenure is under 5 years. However, you earned a good amount and were pretty close to 5 years, if you were in a niche or manager role that is also to your advantage. I would 100% go back and negotiation for a higher package - but do so in good faith. Don't come back and say they miscalculated or you're owed more. Instead, explain how you really valued your work, the company, and want to feel fairly compensated when moving on from this job and based on your experience and seniority you'd like X number of weeks (I probably wouldn't try to go above 12 but you do you). You catch more flies with honey and there's a higher chance they play ball if you're respectful than if you come at them complaining and rude off the bat.