Is a 4DES or 5DES system possible? We know that 3DES is created with $E_{K_3}(D_{K_2}(E_{K_1}(m)))$ to extend DES's key length. Is it possible to extend it further by repeating this pattern? Perhaps using $E_{K_5}(D_{K_4}(E_{K_3}(D_{K_2}(E_{K_1}(m)))))$ for a 280 bit key.
There is a very interesting paper that relates to this exact question (but you wouldn't guess it from the title). The paper is titled Efficient Dissection of Composite Problems, with Applications to Cryptanalysis, Knapsacks, and Combinatorial Search Problems. In Section 3, the paper considers the multiple encryption problem and gives novel attacks that are better than what you would expect (for a general multiple encryption $r$ times). For example, for 2DES there is an attack taking about $2^{56}$ time and $2^{56}$ space. You would therefore expect that 4DES would give you $2^{112}$ time and $2^{112}$ space. However, they show an attack that takes time $2^{112}$ and only space $2^{56}$.

In any case, in practice there is no good reason to use this. You are far better off using AES. DES's block size is too small, and it is very slow as it is (repeating 4 or 5 times will completely kill you).