Thanks for reaching out to the Spotify Community! It's good to have you on board 🙂
Why the songs that I had been downloading just disappeared?
This is an odd one, but the first place to start is by heading over to this link. Once you've logged in here, how many devices are showing? It sounds as though you may have reached the maximum amount of devices allowed for offline use. Does removing these devices and reinstalling Spotify help out with this issue?
Why is it that the Spotify app takes up 4g of storage space on my phone, but I have to download all of my songs again?
When you sync your tracks for Offline Mode, Spotify saves a small piece of data so that streaming is quicker, better and ultimately more efficient. This can take up some storage on your device. However, 4gb does seem excessive. How many tracks do you have synced to Offline Mode at current? Just to note, the steps I've detailed in your previous question should help reduce this data usage.
We hope this helps out, but let us know if there's anything further that we can help with!