It Beta someone tested it and it seems to be equal chance to get any 3 or 4 star hero.
As there are 10 non Season 1 four star heroes:
You will have 10/32 = 31.25% chance for any non S1 four star hero.
And there are 5 non Season 1 three star heroes.
You will have 5/28 = 17.86% chance for any non S1 three star hero.