A quitclaim deed (that is a document where one person gives up whatever ownership interest they have in a property to someone else) is only effective between the parties who signed it. Until it gets recorded at the county recorder's office, it does not appear in the public record, and title companies cannot insure a clean chain of ownership without it.
The good news is that recording a deed is not complicated. Your seller needs the original executed deed, or a certified copy if the original is lost. She takes it to the Cass County Recorder of Deeds in Harrisonville. Missouri does not require a waiting period for recording, so if she walks in with a properly executed, notarized deed, they can often process it same day or within a day or two. Call the recorder's office directly this morning and ask about their current turnaround, because that one phone call tells you whether you have a timing problem or just a stressful afternoon.
If the original deed cannot be located, that is a bigger issue, and you will need a real estate attorney in Missouri to advise you on next steps. I can point you toward how things generally work, but a missing deed with a closing four days out is a situation where licensed legal counsel earns its fee.
One thing worth knowing for future deals: a title search during your due diligence period is specifically designed to catch this kind of gap before you are four days from closing. The strategy guide for land wholesaling flags title complications as one of the principal risks in the strategy. Building a title search into every deal, before you lock in your assignment timeline, gives you room to handle exactly this situation without the pressure you are feeling right now.
What does the seller know about where the original deed is?