Preheat oven to 450F, grease a big baking sheet.
In a saucepan combine water, milk, sugar, and butter. Bring to a boil and stir so the butter fully melts. Remove from heat, dump all the flour in, mix well, should form a smooth dough.
Put back on medium heat. Cook it for another 1-2 minutes to dry it a bit.
Transfer to another container, allow to cool 3-5min.
For each of the four eggs, mix the egg and then mix it into the dough. Mix each one thoroughly. After all four it should be glossy and smooth. It should make a ribon that ends in a V if you lift the spatula. If it's too stiff or doesn't V, add a little more egg (but not a whole egg! A bit at a time.) until it's just right.
Pipe onto baking sheet in 1.5x4" strips. Barely fits on a single big sheet.
Bake 12min at 450F, then turn heat down to 350F for another 20-30min. Make sure not to open the oven door until near the end of the second round. Then turn the oven off and leave the door cracked for 10min for even more gentle drying.
After they've cooled, fill with cooled and set pastry cream. They should take about one recipe worth, injected.
Then pipe a strip of one recipe worth of chocolate icing or ganache along the tops.