Heat 4 cups of milk in microwave for 3-4 minutes just to warm.
In large dutch oven heat butter to melt and whisk in flour over medium-high heat. Whisk until mixture starts to brown and smell nutty. Whisk in milk and continue to whisk until mixture thickens and is smooth. Add salt.
Boil pasta until almost done according to package instructions.
While pasta is boiling stir in cheeses starting with the feta and finshing with the jack. Add in small amounts and continue stirring. The feta may not completely incorporate.
Add the cooked pasta to the sauce and stir to incorporate. If too thick or if going to serve much later add some of the reserved pasta water.