How to wire 4 nm into 2 separate light switches and 2 lights?
So you currently have the black wires from NM4, NM3, and NM1 connected together, and similarly the white wires from NM4, NM3, and NM1 connected together. What you probably have is power arriving via NM4 and being distributed to other rooms via NM3 and NM1.
You can verify this by temporarily disconnecting NM3 or NM1 and observing what parts of your house go dark. (You have already done this for NM3.)
It is also possible that NM4 and NM1 are on separate breakers. Investigate this possibility by temporarily separating the bundles of three black and three white wires, temporarily connecting a load (such as NM3) to each of NM4 and then NM1, and observing which breakers control NM4 and NM1.
NM2 seems to go to switch #1. You can verify this through contact testing, or you can just assume the cable doesn't make any other stops along the way. If this switch formerly controlled the light that you removed then this is a safe assumption.
Now -- to connect your new light and switch #1.

Connect the white wire of NM2 to the bundle of three black wires at the light. Color the white wire black (or any color other than white, gray, or green) with paint or a bit of tape.
At switch #1, color the white wire in the same way. This marks the white wire &quot;reused as constant-hot&quot; at both ends.
Back at the light, connect the black wire from NM2 to the black (hot) wire of the new lamp.
Connect the white (neutral) wire of the new lamp to the bundle of three white wires at the light.

Please be aware that this installation, while safe, is not up to current code, because there is no neutral connection available at switch #1. You are carrying a technical debt, i.e. the future installation of a smart switch at the location of switch #1 might require replacing NM2.
I believe it is proper to replace your ceiling lamp in this way because, in terms of code compliance, you are at least not making things worse.