Shaders Plus essentially is just a collection of all the default shader nodes in Blender, but with new features! These new features being Physically Accurate Thin-Film Interference, Real Time Caustics, and Continuous Dispersion. You use them just like the normal default nodes that we're all familiar with, except now you can get more functionality out of them! These effects help bring you close to the effects seen in industry leading render engines such as Octane, and Redshift.

Have you ever seen those swirly rainbow patterns in an oil puddle in the road, or on a soap bubble, or on a piece of glass or plastic? This effect physically accurately simulates that effect within Blender! This effect also comes with the necessary controls to finely tune your thin film pattern to match real world references! 

This effect comes enabled on all the shaders, Principled, Glassy, Refraction, Diffuse, and Transparent!

These caustics do not rely on refractive light simulation to achieve this look. However, while that can sacrifice realism to some degree, this current effect can get you really far in most cases, especially when even just simple caustics are not at all an option whether it be due to time or budget constraints. This pack solves that for you!

Only Thin Film and Dispersion work in Eevee currently. 

However, Thin Film, Caustics, and Dispersion all work in Cycles natively.

Planned support for Eevee caustics in the future.

So the Shaders Plus v4 are just replacement shaders of the defaults, so it takes the default shaders and adds the Shaders Plus features to them.

The Modules are individual node groups of the Shaders Plus Features, that can be injected into any existing shader setup, even those that are not Shaders Plus based. So if you're revisiting an old project, with the V3 shaders, you'll have to replace the main shader node and rewire and set everything if you want it to have the features. Whereas the modules can simply be slotted in without breaking anything.

The shaders are great for normal workflows especially starting from scratch, and the modules are great for existing node setups to have those features non-destructively.

The Bundle is a discounted combination of the two of these workflows / features.