Try changing dissolve4 to Dissolve(4) it looks like that is the problem.
That is mostly likely correct. It's a transition class and requires a time parameter at a minimum, which appears in parentheses after the class name,
Dissolve. Without a capital (
dissolve), it specifically refers to the equivalent of
Dissolve(0.5), a pre-defined instance of the class — and never takes parentheses. (Ren'Py is case-sensitive.) All that is to say, the most likely error here is the missing parentheses, as you've said. The only other possibility I can see is that the transition was intended to be fast (only half a second long), in which case just deleting the
4 from the end would achieve the speed wanted.