How should I raise my defenses after Act 4?

Armour/ES worked well for me in the campaign. Hard to get a good amount of ES in campaign, so might be easier to stack both. The biggest difference will definitely be resistances. Cap all three, or aim for at least 60% and it should be a pretty big difference. Depending on your skill tree, there is a node around 9 o'clock that adds defences against projectiles directly. That did help me a bit. Just look for a good chest piece with a big amount of armour/ES and rest of your defensive pieces look for resistances first, then whatever extra armour/ES you can get. Rings are also a great slot to get some extra resistances. If you can craft some items, great. But I wouldn't invest too many resources in crafting this early, you can grab some stuff from trade for a couple exalts that will be a big upgrade at a fraction of the cost. I'd craft only if you get a good drop with high rolls and then just exalt slam and go with whatever lands.

You mention the problem and have the answer .. get your resistances up

Thief's torment unique ring carried my sustain for quite some time due to the life gain on hit coupled with oil barrage's hits.
Try also checking for some pieces with armor and armor applies to elemental damage taken that are around your level and that should pair well with the life gain on hit.
Resistances , like others said and you should be good to go.
I leveled the same way when i started the league and this worked better than expected.

Elemental resistance is most important, get it to the cap of 75% asap. Chaos resistance is important too. Then your armour/evasion/energy shield. You probably want 2 lines of element resistance on every piece of gear.

Hey friend, I’m also a newer player here, but I’m at level 82. Had the same issue. Can you give us a general idea of your armor/ES/evasion and resistances as of where they are right now? Should be able to give you some realistic advice with having poor funds as a newer player