Patau, Edward, and Down Syndrome are all the result of either the egg or sperm cell possessing an extra copy of chromosome 13, 18, and 21, respectively. However, what would be the outcome if both cells possessed an extra copy? That is to say, if the fertilised egg cell is left with not three, but four copies of a chromosome?
There are many ways to answer your question, I'll try a couple.
First, extra chromosomes in humans are commonly deleterious. However, there are known rare forms of tetrasomy that are not lethal, see Wikipedia on tetrasomy. Note that the causes of tetrasomy are rare events, so they are typically occurring in just one of the original gametes, not both; for it to happen in both would require a rare event paired with another rare event - not saying this is impossible, just less likely.
Polyploidy in other organisms, particularly plants, is both common and normal, see Wikipedia on polyploidy.