Firstly, welcome to the forum Mihai.
In late April you could still encounter snow up high in the Scottish Highlands; in fact I would say that it is quite likely. The summit plateau of Ben Nevis is a trap for the unwary in bad conditions, as a big overhanging cornice can form over the lip of a gully near the summit, around which the path skirts - potentially very dangerous in the mist.
I have had some epic walks in full winter conditions in April, and on one occasion my wife and I encountered a substantial fall of fresh snow on the summit slopes of An Teallach (Northern Highlands) on the 7th of June!
My advice is to bring an ice axe at least.