Heya!

I've looted more than 400 pods up until now, but I'm still stuck just after the 4th mission - where you recover two fighters from Maas' Corporation.

Is it normal?

Have you successfully finished the mission, like it is explained here?
http://www.i-war2.com/documents/help/faq/faq-acts-0-1
(Act 1 - Mission 4 - The Heist)

If you've finished it, go back to your base, equip your ship with the two fighters and you can do some training with them.

I got a Gunstar mission right afterwards.

I've already completed that mission as described. I still don't receive anything after that.