Yeah, it does sound like quite a high dose. Did your vet weigh him? 4ml would be enough to treat an 8kg bird.
I find if you sort of inject the medication into the bread it works well. Maybe some dried out banana bread would soak up more of it?
You definitely need to give it directly or in a small piece of food that you see him eat. It's not water soluble so it won't work that way. You also definitely need to repeat after 10 days.
We don't have the Safeguard brand here and I'd be fairly sure it's not available in Scotland either but it's basically just the most widely known brand of Fenbendazole in the US. There are lots of other Fenbendazole wormers here with different branding. I just had a look and Panacur is...