Hello andrew.hines,
Thanks for reaching out to us here. Having the option to print a specific T4 slip can be done in QuickBooks Online, in just a few steps. I'd be happy to show you how!
1. Open the T4 slips to Review the information
2. Click on the printer icon in the top right
3. Scroll through the T4 slips to locate the one you need to print
4. On the printer settings, select Custom, in the Pages section
5. Enter the page numbers you need to print
6. Hit Print
You're all set!
Reach back out if you have other questions. We would be glad to help!