I think you can forget about successfully suing anyone over a 4 year old liner being bleached out.
The only possible action you'd have would be against whoever raised the chlorine high at closing, but you'd be hard put to prove in a way that will stand up in court. Plus, the cost of a lawsuit --at least in the US -- is typically going to be more than the cost of a new liner.
For what it's worth, liner color susceptibility varies widely, with solid dark blue being one of the most susceptible colors. It's not a color I'd recommend, for just that reason. Some liners stand up better than others, but I have NOT been able to come up with any method of predicting in advance which liners will stand up best.
Sorry for your problems.
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