Although you state you have new extinguishers now and that are certified for 4 years.
This may be fine in till your next inspection. For your insurance. But technically, you still need to get the extinguishers checked and signed off on a yearly basis, by a BAFE registered company. The manufacturer of the extinguishers will have, this statement printed on it.
Your insurance will find any reason not to pay you out, if you make a claim. To have 3 extinguishers checked annually, it can cost as little as £30 per year and it ticks the box.