Hello, and this is my first guide. Some people seem to be having trouble with missions 4 and 6, so played then and found some ways to beat them.
Mission 4
First, build bank, then forge. Set forge to salvage. Don't worry about the 3 scouts he sends, the base will destroy them. Make a generator ASAP. After a while, the enemy will send a group of marines/scouts. Start making scouts with rally point, and after a while they will die and you will have a group of about 20. Stop making scouts, and make an armory, on to energy. From now on, it's easy and you can attack. Send your group to attack, and make more scouts with rally point off. Just sit back and watch as you destroy him. This mission is quite easy.
Mission 6
Build bank, then outpost. Make a tank from the outpost to clear out some marines that will destroy your partner's scouts. After your scouts/tank are destroyed, put on rally point and make tanks. When you can, make 2 armories and set to reinforcements- now you gain supplies while making tanks. After a while, the attack wave is destroyed, this mission becomes extremely easy. Upgrade to barracks, and send your tanks to attack. Rally point about 10 groditz, and then send them out. You can also deploy a mod phantom if you want, because it's communist, I think, and you get the purple stuff from killing. From this point on, turn off rally point and just watch as your groditz, tanks, partner's phantoms/sakatas own everything. This mission is a quite a bit harder than mission 4, but mission 5 is still harder than this.
I'm still not quite sure of a way to easily beat mission 5, but hope these 2 guides help! Thanks for reading.