What is the difference between 必须, 得, 必需, and 需要? I know that all of them mean "to have to" in some form or another, but are they interchangeable?
First let's clarify the definitions of 須 and 需
須： Adverb/verb, meaning should, must; 《必須，必定，須要》

你（必須）參加這次會議
You must participate in this meeting.

需: Compound, meaning need, require; example compounds: 《需要(require)，需求(demand)，必需(need)，急需(urgent need)》

植物生長（必需）陽光和雨露
Plants require sunlight and raindrops to grow

It is very easy to mix up 必須 and 必需, even for native speakers, as they have the same pronunciation (cantonese and mandarin) and very similar meanings. In english, both can be represented by &quot;need&quot;. &quot;You need to participate in this meeting&quot; and &quot;Plants need sunlight and raindrops to grow&quot;.
The easiest way to distinguish the two is to see whether it is a noun or a verb that comes after it. If it is a noun, use 必需， 需要; if it is a verb, use 必須.
得 is quite interesting because it can be used to replace both 必須 or 必需

有錯誤就得批評;做得好就得表揚
Wrong doing needs to be critiqued, good work needs to be complimented
這公工程得三個月才能完成
It takes 3 months to finish this construction

Note that 需要 can be used as a noun as &quot;need(s)&quot;