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5th does not get a CL spot if another team wins it

Liverpool is in top 4 and if they win the champions League I think the 5th team will qualify.

I'm sorry but you are mistaken.

So what happens if Liverpool wins champions league and finishes top 4?

IF the team which wins the Champions League in the 2017-18 season also qualifies via their domestic league, then their place will be given to the champions of the association ranked 11th by UEFA - Czech Republic.