Think you are looking for a Preston Latex 4M Competition keepnet. also has large holes (not carp mesh) but you can almost double that 50 budget. Most river nets like daiwa and so on are 3.5m. Best bet would be Middy. They have a massive selection.
Sensas Rotterdam keepnet, available in 2.5, 3 and 4 metre lengths,
http://www.barfordtackle.com/asp_pages/products.asp?id=621 I realise that this has a carp sack base at the bottom, but nearly all keepnets do these days. Ant the models that don't, advertised as silverfish nets are usually under 3.5m in length.
MIddy don't produce a net that is 4m long, and I think you will struggle, apart from the above.
P.S. I have no connection to the link, just was the first link I came across.