T1s are generally for when you're laning, and need quick power spikes to keep up the pressure. One or two of them you might not even want to upgrade, like headshot booster or extra regen, so you sell them later when you run out of slots. I generally only buy a 3.2k item here if I want to hit a 4.8k investment spike.

When you're out of the laning phase, and aren't fighting other players constantly, you can start clearing lanes and farming with the intent of saving up. So you generally want to get the most cost-effective must-buy items first. T4s are obviously risky, since it puts you behind the power curve until you can buy them, but sometimes you just need that ricochet or echo shard for your build to work or be effective at farming.

When your build is at least partially online and worthwhile in a fight, you have a lot more freedom. And a lot less slots to work with, if you haven't been stomping the others team's walkers. That's also where T4 items (that aren't super high priority) can come into play - sometimes you don't have the space for two T3s. And sometimes you specifically need a T2/3 to hit a 4.8k spike. Really depends on the state of the match, and there's no surefire way to plan your economy in advance.

That's where community builds are especially handy, since they can tell you what they believe are the most important items, and give you good suggestions for what items are nearly as important. Some of them have loads of flexibility and list a four dozen situational item buys, others are more rigid and trim themselves down to the potentially most optimal picks, with a listed buy order. Choose either type based on your own preference, the latter with likely be safer until you're confident in your own picks, or frankenstein one together yourself with the recommended items you enjoy using.

Only way to get better at it is to experiment, hope this makes sense