Hello Graham,
At 4 years old you must remove or replace the filter, You should check the manufacturers instructions but I think you will find you are well past the suggested service life of the filter. It might actually be more of danger now from what it has caught and harboured for so long.
You question about remove or replace is in my opinion dependant on where you caravan. For example if you only travel in the UK, then the quality of water supplies are well regulated and you shouldn't need a filter at all. But if you travel abroad where water supplies may be less well managed, then a filter has its uses.
But there are places even in the UK where the supplies are more heavily chlorinated and that can make teh water taste unpleasant. A filter can help to remove such taints, but you only really need that for drinking, so a filter jug is probably the best solution.
If you have an in line filter, your local caravan dealer should have the parts needed to replace the fitting with a section of pipe, If its a water inlet filter, then as Sea Dog suggests it may as simple as removing the cartridge from the screw plug. If its a plug in cartridge type, you may find an empty one designed for flushing.