Would rendering After Effects project in Premiere Pro decrease quality?
Importing from AE using dynamic linking and rendering in Premiere or Media Encoder will yield the same quality, but it does give you a little less control over the render settings. 

While in AE you can choose whether to render with frame blending , motion blur, draft settings and so on, Premiere will always render your AE comp with the current settings. So when you're setting up your final render in Premiere, make sure you jump over to AE and turn on motion blur and frame blending and check that all your layers are rendering at full quality, not draft. Most of the time you won't want these on while you're editing because they slow down the preview so much.

As far as quality goes I believe that it uses the same render engine, so don't obsess about it. In fact re-encoding from an After Effects render is probably more lossy than importing using dynamic linking, because of the two compression stages. But you will never ever notice the difference one way or the other, trust me. Any miniscule losses due to things like quantisation errors and so on are going to pale into insignificance with quality lost in the compression to MP4. 

Use the way that makes it much easier for you. For me that's importing via dynamic linking, allowing me to change things in the AE project without re-rendering all the time and having great big render files all over the place.