The Poisson distribution is a single-parameter distribution, the one parameter being the mean.
Basically, the general idea behind a Poisson distribution is that for any size area, you have the same process intensity. For example: let's say you're counting the number of people that come into a store. If the average number of people is 60 per hour, and the distribution is Poisson, this is exactly the same thing as saying that one person per minute enters the store, on average.
Therefore, you simply need to scale your average to match your units. The average number of algae is 2 per liter, which turns out to be 2 per 1000 cc, which means you have .004 algae per 20 cc, on average. So, you've done that part right.
Now, you can use your Poisson distribution formula using this process intensity parameter.
Another way of looking at it is this: if you have a 1 liter volume that has 2 algae in it, you can draw a neighborhood around one algae that has a volume of 20 cc; if you draw neighborhoods at random, what is the probability you enclose an algae?
So, to answer your question, you need to compute $P(X > 3) = 1- P(X \le 3) = 1- (P(X=0)+P(X=1)+P(X=2)+P(X=3))$.
Compute $P(X=k)$ using the PMF with $\lambda = 0.04$: $P(X = k) = \frac{\lambda^k}{k!}e^{-\lambda}$