you have already learned a valuable lesson. 4 days is far too long for a poor bonsai to stay home alone.
First aid for dry bonsai:
Immerse the pot in water as soon as possible. Dry soil can be hard to rewet so leave it soak overnight.
Return the tree to its home and continue care. It may need less water while there are no leaves to draw water but keep soil damp.
Keep fingers crossed and all other usual means to recover from tragedy.
If you have caught it in time new shoots will emerge from branches or trunk or roots depending on extent of the dehydration. New shoots may take up to a month or 6 weeks so don't give up hope too soon.
If it does not recover save the pot for a new bonsai.