In fact, 1 mk 4 ship is better than 4 mk1 ships(that sustained DPS thing), and will actually be able to take out 5 mk1 ships and still survive: Explanation below, with algebra.
With attack, armour, and health all scaling linearly, the short answer is that a mk n ship will fight evenly with (sqrt(1+8n2) - 1)/2 mk 1 ships, and that a mk n ship will fight evenly with sqrt(1 + 8(n/m)2)2 mk m ships. To see this, take a mk n ship vs x mk 1 ships: Armour can be assumed to be zero, and attack and health both 1(dps) at mk 1. When the first mk1 ship dies, the mk n ship will have taken x/n damage. At the second death, the mk n ship will have taken (x-1)/n more. So, to take out the mk n ships, you need (1+2+...+x)/n to be at least n. This gives the equation x(x+1)/2n = n, or x2 + x -2n2 = 0, and by the quadratic formula, x = (sqrt(8n^2 + 1)-1)/2, and this generalizes to the mk n vs mk m case.
The +1 actually doesn't make all that much difference at the numbers we are talking about(even mk 5 vs mk 4), and so it's almost the same as 1.4n/m - 1/2.
If you are defeating the enemy piecemeal it gets even better, as a mk n ship can take out n2 mk1 ships if it gets to fight them one at a time.
Since this is my first post, hopefully I haven't embarrassed myself too much by making silly errors here. Also, I really like this game, as can be seen by the effort to calculate this out. Well done.