The house we bought has mature landscaping but the builder cheaped out on filling the front garden bed all the way up with soil. There are mature plants all throughout but I can also see bottom of the concrete walkway to our front door the entire way to the driveway.

My worry is if I add 4" of soil to this area (about 200 sqft of space) it will hurt the existing plants. Should I be concerned or will this work itself out?

I also have two downspouts coming into this area and I'd love to get that water out of there with more piping but it would just dead end into the concrete walkway.

Any thoughts or suggestions?

Mulch it an inch or 2 each season until you get to your desired height. Plants will grow above it each season.

As for drainage, a mini French drain that pops up at the concrete to bypass the drainage water from going over/through your beds.

The issue for my beds is that we use pine straw as the weed barrier here in our neck of the woods (North Georgia). I lay it on thick but it packs down over time.

I guess I could always pull back all the pine straw and add the mulch like you said and still put down pine straw after but that feels like a ton of work

Mulch is the answer and would be the same work as adding dirt. Mulch/Dirt then top with pine straw. However, mulch will compact, so if you want to do it once, just use the dirt. If you use a quality landscapers/rose mix, it drains very well and should not impact the mature plants. they will grow roots it the built up area and grow out of it.