Let's evaluate each proposition step by step:

(1) An increasing function means that as $x$ increases, $f(x)$ also increases. This does not necessarily mean that the graph of $f(x)$ will intersect the $x$-axis; hence the equation $f(x) = 0$ may not have a solution. For instance, consider the function $f(x) = x$ for $x > 0$. Proposition (1) is incorrect.

(2) A decreasing function means that as $x$ increases, $f(x)$ decreases. The graph of such a function will intersect the $x$-axis at most once, because having more than one point of intersection would contradict the monotonicity of the function. Therefore, the equation $f(x) = 0$ has at most one solution. Proposition (2) is correct.

(3) If $f(x)$ is an even function, it means $f(x) = f(-x)$. If $x \neq 0$ is a solution to $f(x) = 0$, then $f(-x) = 0$ as well, providing us an additional solution: $-x$. However, if $f(0) = 0$, this would give us an odd number of real solutions, including zero. Hence, it is not guaranteed that an even function will have an even number of solutions for the equation $f(x) = 0$. Proposition (3) is incorrect.

(4) An odd function satisfies the property $f(-x) = -f(x)$. If there is a solution $x = a$ to the equation $f(x) = 1$, then we can say $f(a) = 1$. By the definition of an odd function, $f(-a) = -f(a) = -1$. This implies that $-a$ is a solution to the equation $f(x) = -1$. Proposition (4) is correct.

Thus, the correct propositions are (2) and (4). $$\boxed{(2)(4)}$$