So having read through the decision, I think this works as a quick summary of why it was 4 points:
Commission found that the "entry point" for any "significant" breach of the spending limits is 3 points
In Everton's case this was increased up to 6 points for two aggravating factors: One being the scale of the breach (£19.5m, or 19% of their threshold), the other being that they had been found in their appeal to have supplied some "incorrect" information to the Premier League (which the Premier League originally presented as Everton "misleading" them. The finding of "incorrect" rather than "misleading" is important).
In Forest's case, the aggravating factor was solely the scale of their breach, albeit this was both numerically and proportionally much higher than Everton's (£34.5m, or 57% of their threshold; alternatively, a breach 77% higher than Everton's).
While the Premier League argued that this should correspond to a 5 point increase in the deduction, raising the points deduction up to a total of 8 points, the commission disagreed.
This was in large part because the Premier League has no "fixed formula" for points deductions based on the size of a breach, and that an insolvency event would result in a 9 point deduction. There's a lot of explanation for why they came to this decision, but effectively, the commission concluded that, as Forest's breach was a "significant" breach not a "major" breach (i.e. one involving insolvency), the absolute upper threshold that any club who makes a "significant" breach would expect (without major and unusual aggravating circumstances) would be 8 points, as insolvency would represent the absolute worst case scenario for failing to adhere to profit and sustainability. They also concluded that Forest were unlikely to be the largest ever Premier League breach, and therefore they shouldn't be punished at the absolute highest end of the scale.
Instead, the commission decided that the level of breach should correspond to a 3 point increase to the deduction, raising to total deduction to 6 points.
Forest were found to have had two mitigating factors in their favour: that they admitted the breach early on, and that they complied with the Premier League. As such, the commission reduced their deduction by 2 points, taking the total deduction to 4 points.
So, basically, you can overspend by as much as you want in the Premier League, and as long as you don't do anything too egregious alongside it, you'll lose no more than 8 points.
