We would be grateful for a means of "fast entry" of materials in order entry. When creating a sales order, after entering the customer number, a list of materials is displayed, along with the quantity of the customer's last four orders of each product. 
Is there any easy way for GuiXT or InputAssistant to replicate this functionality?

Yes, this is possible. Create an entry table ("Table" statment") and read the previous customer orders with a "Call", function module /guixt/dbselect. Implement the update via an InputScript that automates VA01, or via the BAPI for order entry.