Need filtered Drop down list to show options based on date entered in multiple fields

Drop-down lists are either a manual list, or from a related table. I'm not aware of any way to set the list based on other fields in the same table. However you could use Table to Table imports to set up a related table like this:

1. Create a table called, say, Order Vendors, with just two fields: [Orders] & [Vendor Name]
2. Set up 4 Table to Table imports that copy the Order Number and Vendor from your ORDERS table to the Order Vendors table. Import 1 copies  [Order Number] & [Vendor 1 - Name], Import 2 copies [Order Number] & [Vendor 2 - Name] etc.
3. Then create a relationship where Order Vendors is the parent of ORDERS. Keep the Reference Field set to [Related Order Vendor]. Set the first (proxy) Lookup field to be [Vendor Name], and the second Lookup field to be [Order]
4. Once the relationship is created, change the ORDERS field [Order Vendors - Vendor Name] to your preferred name ([Vendors in Order]).
5. Make the [Related Order Vendor] field conditional on the [Order Number] = [Orders] (see Conditional values in the field properties.

The clunky thing with this solution is that it requires an admin to run the Table to Table imports - this is easy to do once set up, but not ideal. I'll have a think about a better solution for you....