Yes, Jerry. You can bill your customer for a single line item from an estimate with multiple line items through the Progress Invoicing feature in QuickBooks Online (QBO).
When you convert an estimate to an invoice, you can choose to invoice for the Remaining total of all lines, Percentage or amount, or Manual entry for each line. This is especially useful if you have completed only part of the work and want to bill accordingly. Refer to the screenshot below for reference:

Since you have an estimate with four line items and have completed only one, you can select the Manual entry for each line option when converting the estimate to invoice to bill only the completed line item.
Please ensure that the Progress Invoicing feature is enabled in your Account and Settings.

After that, proceed to bill your customer for the one line item. For a detailed walkthrough, here’s how:
- Navigate to the Sales menu and choose Estimates.
- Find the estimate with multiple line items that you want to invoice from.
- Click the Create to Invoice option.
- Choose the Manual entry for each line.
- For the line item you wish to bill, enter the total amount.
- For the other line items, enter zero or leave blank.
- Select Save and Close.
The new invoice will generate with only one line item billed. While the other items still appear on the invoice, please note that only the chosen line item will have an amount reflected, and the rest will show no charge. Kindly review the invoice details and add any necessary notes. When you are finished, you can Save and send the invoice to your customer.
You may also find this article helpful if you want to modify your sales forms to reflect your brand’s unique identity: Customize invoices, estimates, and sales receipts.
If you have any other questions or need further assistance, reply or create a new post, and I’ll be ready to assist you.