You don't need to sacrifice (tribute) any monsters to summon a level 4 monster. Level five monsters require one sacrifice.
You need one tribute for a Lv5 or 6 monster, and two tributes for Lv7+, though many higher level monsters have additional or different requirements.
Unless they are monsters that are special summon only, level 7+ monsters require 2 tributes.
You only need 2, like any other tribute monster that's level is 7+
It depends on the level. If it is 4 stars or under, then no. If it is 5-6 stars, then you need to tribute 1 monster. If it is 7-12 stars, you need to tribute 2 monsters. Many high level monsters cannot be tribute summoned though and will have their own summoning text.
It is Level 5, so one monster.
You need to tribute one monster for the tribute summon of a Lv5 or 6 monster, and two tributes for any that are Lv7 or higher, even Lv10 ones like Yubel. Additionally, some monsters require three tributes (Obelisk the Tormentor, for example), but that is stated on the card, and it not related to the level.
In the Yu-Gi-Oh rule book it states that an 8 star monster only needs 2 tributes.
As per the basic rulebook, you need two tributes for monsters of Lv7+. Some high level monsters have rules text to say they require three tributes, but these are exceptions. In the case of monsters like Yubel (Lv10) they still only require two tributes.
The Supremacy Sun is a Lv10 monster so requires 2 tributes.
It depends whether or not you are a member and what your monster's level is.
It doesn't have a set amount of monsters needed. You just need to have monsters in your hand or on your field whose levels equal up to 8 stars. For example, if you had another level 8 monster in your hand, you could simply use that one monster as the tribute. Or if you have two level 4 monsters (either both on field, in hand, or one in hand and one on field). Also, you can go above the 8 star total, such as using a 5 and a 4. As long as you have at least 8 stars being tributed overall, it doesn't matter.
You need to tribute 2 monsters to summon a Lv7 or more monster, so you need 2 for a 'Blue-Eyes White Dragon'.