I've had Now Broadband for a couple of weeks, and have been having issues with 4OD buffering on my Smart TV. It's virtually unwatchable most of the time. Netflix and Prime are fine and iPlayer is mainly OK, but had issues one day. 4OD worked perfectly with my previous broadband, so definitely something to do with change of provider.
TV is in same room as router, in direct line of sight. There isn't a practical way to run an ethernet cable to it. Speed is normally 30-32 mbps (right at the minimum guaranteed speed, which is a bit disappointing). Can't see details of speed on TV, but appears to have reasonable wifi connection. Nobody else using other devices at same time.
Because you mentioned you have a Netflix subscription.
You can check the download speed on your Smart TV by opening the Netflix App, then go to help, then click on check Network.
I find the download test results from Netflix pretty accurate from my experience.
With regards to the All4 App on your Smart TV perhaps try a fresh App download and if your Smart TV supports secondary DNS, then try something like the Google Public DNS on it.
I use Google DNS on my Smart TV with pretty decent results using all Apps with NOW Broadband.