but I can't find anything past level 4. What gives? I have loads of money but nothing to buy.
Not everything can be found on the beaten track. Some systems have planets that are not well known. Here is a hint: Press F3 to bring up the inventory screen, click News Console, and then click System Info (or something like that). If the system information indicates rumors about a secret weapons facility or an unknown planet, pay attention to them. They could be true

Some of the best stuff in the game can be found at those secret locations. Think of the cool stuff in EL like trying to buy an RPG or a tank in the real world. You can't pick one up at your local mall; you must look around in out-of-the-way places for it.
Also, try completing the beacon quests if you have not already. The areas you must traverse, particularly in the Renegades quest, could be worth your while and lead you to new locations. You can also explore deep space, as there are undiscovered systems out there (so I've heard. I've not found any yet). In short, once you get a hang of the main systems, you will need to get off the beaten path to find the cool stuff. The stuff off the beaten path is what can really make Legends shine.
Finally, if you are bored of fighting identical fighters, join MP! No human pilot is identical so you will have a blast (maybe literally) with the challenge of fighting a real human pilot. If you want to stay with SP for now, try completing 4 or 5 waypoint missions in the warzones. Or maybe take a 4-5 waypoint mission in a place like Aries and challenge yourself to not rearm or repair during the mission. Maybe complete contracts using only guns. There are ways to make combat more challenging.
-Accountant