Anyone know how to add multiple addresses for a single customer. I have a client with 4 seperate addresses.

Welcome to the Community, @mattcraker.

You can only add one address per customer in QuickBooks Online. However, you can use sub-customers so you can set up different addresses. 

First off, you'll need a parent customer (top-level) to create unlimited number of sub-customers. You can set that existing client with multiple addresses as the parent. Then, link three sub-customers to it with different addresses to make it four. Here's how: 

1. Go to the Sales tab in the left menu. 
2. Select Customers.
3. Click New Customer at the top right.
4. Enter and complete the field in the Customer Information window. Since you can only use the customer's name once, you can add a number or symbol to the name. Example: Customer1. 
5. Select the checkbox for Is sub-customer.
6. Choose a parent customer in the Parent▼drop-down menu, 
7. Uncheck the box beside the Same as the billing address.
8. Enter the specific address for this 1st sub-customer. 
9. Select Save.

Repeat these steps to set up your 2nd to 3rd sub-customers and enter different addresses. 

You now have four different addresses for your customer. When you're ready to create an invoice, you just need to choose either the parent, a specific sub-customer or whichever has the correct address that you intend to send the invoice to. 

I'm also adding help articles below that you can read through:

• Manage your customer list. It provides more information about creating sub-customers in QuickBooks Online. 
• What does the bill with parent feature let you do on invoices? It helps you learn about the benefit of having customer and sub-customers. 

If you have any other questions, please don't hesitate to message us. We're always glad to help.