Four days under the bed is completely normal — please don't panic. Ours hid for nearly a week. What helped: confining her to ONE quiet room first with all her stuff, rather than the whole strange flat, so she had a small safe base to expand from.
My cat has been under the bed for four days since we arrived and I'm quietly heartbroken and worried. The flight and the new smells clearly rattled her. For those who moved with an indoor cat — how long until they came out and acted like themselves again, and did anything actually help speed it up?