Are you sure it's DOT5 you used? It's surprisingly expensive and difficult to come across by accident - it's the silicone based one. Every other DOT is glycol based.
You can buy DOT 5.1 fluid in blue colour - which is glycol based and safe to mix with DOT4.
As above though, if you put some of (what you think is DOT5) in water, it should separate and sit on top of the water. if it's glycol based, it'll mix with the water and not separate.
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The rain kept a trollin'
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