Lots of Prestige level answers for a L4 player
Yeah, that was funny. In fairness, there was some info sprinkled in that was applicable to a level 4 or pertinent to the long-term decision making, but funny nonetheless. As a level 4, keep in mind you will only stay at that level for a very short period of time in this game. Even at a slow pace, you will be at level 6 in a very short period of time. A lot depends on whether you want to make a lot of turret swaps later on, when you hit level 8. The game changes a little through level 7 and significantly at level 8. That's why a lot of the Prestige players chimed in with higher level feedback.
As DarkRebel said, at level 4 you will see a steady diet of Wookiee and rebel soldier attacks. A mix of bursts and mortars would work. You will most likely swap out of the mortars and rapids at level 8, so no sense in having rapids. That will minimize the number of swaps you need to make. Bursts are better than rapids at level 8 and probably just as effective as rapids at the lower levels. Most rebel units don't have the range to hurt bursts, and most rebel players don't really know how to use hails even when they get them. So many need to be taught how to use buffers, deployment timing, and the range differentials to get the right spacing. I would only keep 2 mortars, since you will swap out of those too.
Since I suspect this is really a "how do you improve your defense" question, I will add that you keep snipers, togs, bikes, MTT's, ITT, or Orbak in your squad center for defense. Learn to 3 star on offense without any squad center help, so you can keep your squad center troops intact. Those alone will stop most of your attackers at level 4-7. Get your squad center under shields and inside your walls, and that more than turret selection will keep your base protected at the lower levels. Upgrade your walls as a priority, so keep a droid free to upgrade your walls every time you have enough alloy. At level 7 and below, it's nothing but ground troops. Walls are important.
A subtle strategy would also be to delay adding turrets. Either full blown mini base or delay adding turrets and hold off on traps. For example, don't add your level 5 turret allocation until you go to HQ level 6. The problem is the transition. When you get to level 6 and immediately add your level 6 turrets, the game matchmaking will treat you more like a level 6 than a level 5. Your troops will still be at level 5 at best. That matchmaking algo includes turrets and traps in your base score, on both offensive and defensive attacks, so you lose ground on offense and breakeven on defense at best when you add turrets and traps. Sure way to struggle.