I am stuck on the fourth level (at least I think) of Yager. You have to mark all of these missile turrets and I can't do it. I've tried heaps of times. Does anyone know how to do it or have any cheats that can skip the level or make me invincible?

I'm also stuck on this level!!! It's a bit further than level four though, about number eight I think. It's so hard!

Hawk if you manage to do this level let me know how please!

You want to be specific with your problems?
To mark them, switch weapons to the marker (the right most weapon), once you're in range of a missile launcher, the cursor should hover over it, hold fire till it's marked.
Stay in hover mode, and stay as low as possible. Getting too close to the launchers or gun turrets let's the enemy know you're there.
The missile launchers are the bright yellow objects on the radar. Do the first 4 in front of you, then there's 1 to the left. Then further to the right of the first 4, there are two on an island, and two or aroundabouts where the tall building is on the hill.

These seem to be the only ones you can mark without the enemy spotting you, so just switch to jet mode and fly to the bright yellow dots on the radar and mark all the remaining ones.
Then after that, stick around the carrier and just blow shit up.

after you've marked them all, if you are running short of health i like to hover below your ship and when you need to go elsewhere just jet through and do it, then return, plus near the second "mark the missiles" bits there is a repair point.

Once all of the missle platforms (marked in bright yellow on your radar) have been targeted using the marker, just stay alive. there is a repair bay where the submarine starts off by the docks along with some weapon ammo of various types. Take out the pirate pilots and tanks and you will then have to mark the bridge that has come down. The captain will order this blown up and this will become your checkpoint. Use the platform to repair your ship before moving through to the next area, take out as many pirate ships and tanks as possible before being asked to blow up the mines in the water. Once the mines are out of the way there are 6 or 8 mobile missle platforms that have to be marked as well. Be careful as these mobile sites will lock on to you and take you out. there is a repair platform behind two of the more static mobile missle platforms. You will need to fly around taking out more tanks and pirate ships until you are asked to blow up some more mines, you will be able to attack the submarine, but after a while the DST will come to the rescue of the pirate sub.

If you guys think this level is hard, then play the one after it, flying around canyons and then having gun turrets fire at you the moment you raise your head up out of them. Thats a harder level, but the worst bit is having to find the control room. Good luck with it and drop me a line if you need any more help.

I don't have the patience. I let the pirates fight me - you can hold on long enough, especially with the time you get.