I don't know what kind of tips I could give you (if any). I'm not really good at explaining that stuff.
What path do you take in Mission 4? I can take the top path with all those fucking flying beasts and mostly clear the level on one credit. I believe the only times I've died near those guys was because I didn't jump far enough.
The bottom path isn't too hard athough sometimes I get a cheap death from one of those slugs. The zombie part is also pretty easy if you kill them right. I usually jump knife the zombies that lower down before they can even attack. And when the zombie dogs all attack I just bounce on their backs to avoid turning into a zombie myself, then I knife 'em.
Or are you having trouble with the boss? I can clear the boss easily now. I used to have trouble with the ghost beast type things but now I know the timing of their blast. I find the yellow and red laser avoiding easy.
It's kinda tough on the Xbox version because I've had to get really good using the SCII arcade stick.