I had the same problem even before the release of 0.4.
I follwed these steps:
Step 1: Lower the textures of your modded vehicles from 4k to 2k with OpenIV. You might have to download textures for your cars that are only in 2k. Sometimes only this step is enough to fix the problem, for me it wasn't enough though.
Step 2: If you're using VisualV, Radiance or something similar you might have to lower your graphic settings ingame just a bit. There are two specific options I had to adjust, but I don't remember which ones, unfortunately. I might have to check it out again. Anyway, for me that was the solution even though my computer should in theory be able to run the game on ultra well.
Step 3: Some people suggested to run GTAV on "high priority" CPU settings through the task manager. It might have worked for some, for me that created other problems like the game not even starting, so I'd be careful with that. But since some people said it helped, I wanted to give you this option, too.