What's the difference between 否则, 要不, 不然, and 要不然?
否则 is often time more formal and comes across as more strongly worded (directly admonishing the listener). Much like saying "you need to do X, otherwise/if not, Y will happen (as a consequence)."

E.g., 今天你必须来参加会议，否则后果自负

要不 comes across as the softest (has the same as either/or, in the same manner as a suggestion), but it might the nuance of sounding as if one is being admonished, depending on the context. Much like saying "you (either) need to do X, or Y might happen." 

E.g., 今天你必须来参加会议，要不老板会生气的 

However, note that 要不 is (more commonly) used to as a way to directly or indirectly ask the listener for consent (based on the context it's used). Seeing the 吧 typically will indicate that this is a suggestion.

E.g., 一个人看电影没意思，要不我们俩一起去看吧. 

不然 is often used when you want to assert a matter of fact (typically in a neutral manner), the manner of speaking is probably somewhere between 否则 and 要不 in terms of strength. Much like saying "you need to do X otherwise Y might happen."

E.g., 今天你必须来参加会议，不然老板会生气的

要不然 is a about the same as 要不, but in a bit stronger tone than 要不. Much like saying "you need to do X， or otherwise/or else Y might happen." In my opinion, 要不然 can be used interchangeably with 要不, but only in when used in the context of "otherwise."

E.g., 今天你必须来参加会议，要不然老板会生气的