Hey,
i will try to explain as much as i can. You can find every needed information in material's datasheet. Which are mostly DK, Df, Td and Tg.
1) If your material added in the Altium's database you just suppsoed to pick it and the values loaded directly. If you can't find your material in the list then there is no need to type the exact name. You should fill the values. In Altium designer or their website you can chech how the calculations made and how the constants affect in various considerations.
2) The material is a prepreg so it does have a dielectric constant. you will have layers above and below of it and it's purpose is to seperate them (with respect to it's strongness which is Dk.)
3) The "strongness" of the material differentiates under different frequencies. So you should consider the Dk value with respect to your signals' frequencies. I would use 3.38 because I assume that you will use striplines but it could be asked to firm.
4) Df is affects loss in signal simply. And mostly effective on high speed (~10Ghz) signals mostly. But just type it down in the related column. Not a big deal.
I am leaving "5" to someone else.
I hope above would help you a bit. You can chechk this link:
FR4 is by far the most popular set of PCB substrate materials. Here is how FR4 dielectric constant and your material properties will affect your next PCB.
Take a look at our comparison of PCB materials for your next high speed or HDI PCB.
In this OnTrack episode we'll talk about Dielectric constant (Dk) and the difference between Design Dk & Process Dk. How datasheets can be confusing and can impact your design? Listen in for more.
Have a good designing.