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I am getting a strange noise when I go into 4th gear, it happens when I start to speed up and also when I release the clutch after I am at 60 it disappears and only comes back if I speed up. No other gears do this.

It sounds like a strange rattling noise. Not sure if it is the gearbox or the exhaust perhaps the muffler but it sounds like its coming from underneath the car.

It is fairly consistent and it isnt affecting the performance of the car.

I cant see anything underneath the car that could be causing it.

Any ideas what it might be?

First thing that occurs to me is that it's one or more of the exhaust heat shields, however usually this can occur in any gear, but only during a certain rev range, i.e. 2000-3000 RPM. There's a exhaust heat shield at the headers, cat, & bullet muffler toward the rear of the car. The most common shields to make the 'zinging' sound are the header & bullet, which occur due to obscured metal tabs breaking, allowing vibration.

When the car is cool (obviously) grab and try to shake each of the exhaust shields under the car. My previous exhaust made these noises and I crimped it slightly with pliers on the offending rattling piece and it was quiet.