uDoes each level contain material from the previous levels?v
Because the JLPT is a language proficiency test, essentially, yes, higher level tests contain concepts that are also present in lower level tests. Therefs really no way they couldnft. If youfre designing the lowest level of language test possible, it has to test whether the person taking it understands the most fundamental concepts of the language, like the correct way to use pronouns, how to form sentences, how t conjugate verbs, etc. Those then become the building blocks for more complex aspects of the language, so they canft be jettisoned from higher level tests.
To give you an example, letfs say we were designing an English proficiency test, and for the lowest level we wanted to see if the test taker knew the difference between gIh and gme.h So maybe wefd give them a question where they choose between them, like:
u(I/Me) played baseball yesterday.v
Now letfs say we want to make a question for a higher level test to check if the test taker knows how to use the word gwhenh as a time marker. So maybe the question we design is:
uI played baseball (when/at) I was in high school.v
And moving up one more level, now we want to see if the test taker can use conditional tenses, like gwould have,h so we make a question like this:
uI (would have/will have/have) played baseball when I was in high school, but my school didnft have a team/v
As the questions go up in difficulty/test level, they incorporate more and more of the concepts from lower levels. So while the third question isnft necessarily testing whether or not the test taker knows how to use gIh and gwhen,h itfs taking that knowledge as a given, and without it the test taker probably isnft going to be able to answer the question thatfs checking for their understanding of conditional tenses.
That said, the specific knowledge the JLPT questions are designed to test for generally match up with their specific test levels. Using our English examples above, the highest level test isnft going to throw in an easy uI played baseball (when/at) I was in high school.v question that test takers can essentially get a free point for, and the same goes for the JLPT. So in broad terms, yes, the material for JLPT N5 will show up on the N4 test. However, your N5 knowledge probably is only going to go so far as letting you understand the N4 question, not figure out its answer (figuring out the answer requires N4-level knowledge).
uFor example if someone was to study jlpt n4 would they also be covering material from jlpt n5?v
If youfre working with materials specifically designed to prep students for the N4 test, the lessons probably wonft be explaining N5-level concepts. The N5-level concepts will be present as the building blocks of what the N4-lessons are dealing with, but explanations of the N5-leve things probably wonft be explained. Again, to use our English examples, if the lesson is teaching that the proper sentence is gI played baseball when I was in high schoolh and not guI played baseball at I was in high school,h itfs probably not going to take the time to explain that gI played baseballh is correct and gMe played baseball is not.h
Honestly, though, if youfre at the stage of your Japanese studies where youfre still getting ready for the N5 test, itfs probably too early to worry about studying for it specifically Itfs the lowest level test, and so it covers the most basic concepts of Japanese. Right now, youfll probably make better progress simply taking an entry-level course or working through an entry-level self-study book. If youfre starting from zero, everything you learn will be helpful, and in the long term youfll make smoother progress if you start by getting a solid feel for the fundamentals of the language, not by immediately focusing on whether or not something will be on the lowest level test.