Enhance edges in OpenCV (Java) while removing noise

You want something like this?

Noise is usually present - it can be reduced - you can distinguish the circles with adaptiveThreshold and then find the circles using the method I described below. The point is, the real-world image you get from the camera may contain a whole bunch of other circles - so it might be best to find all the circles. Compare them all in size. Find the 4 circles that are most similar to your marker in terms of color, size and placement.

A quick python code to distinguish the circles:

# Make a gray version
gry = cv2.cvtColor(im, cv2.COLOR_BGR2GRAY)

# Thresh
ada = cv2.adaptiveThreshold(
    gry, 255, cv2.ADAPTIVE_THRESH_GAUSSIAN_C, cv2.THRESH_BINARY, 11, 2
)

# Remove noises
out = cv2.medianBlur(ada, 7)

# Invert colors (Not needed)
out = ~out

I tested the Python code and it works; you can find an equivalent for Java or C++. I tried to explain the Java code but I wrote it on the fly and did not test it. The Java code I wrote probably has errors, but it does get the point. With a bit change will probably work. I also wrote the code that should find the circles as the last block. Working with it is tricky and requires adjusting the parameters.

Java:

...
Imgproc.cvtColor(im, gry, Imgproc.COLOR_RGBA2GRAY);
Imgproc.adaptiveThreshold(gry, ada, 255, 
    Imgproc.ADAPTIVE_THRESH_MEAN_C, Imgproc.THRESH_BINARY, 11, 2);
Imgproc.medianBlur(ada, out, 7);
...

and for finding circles:
Java:

...
SimpleBlobDetector_Params params = new SimpleBlobDetector_Params();
params.set_filterByCircularity(true);
params.set_minCircularity(0.01f);
params.set_maxArea(50000);//important
SimpleBlobDetector detector = SimpleBlobDetector.create(params);

// List of detected points 
MatOfKeyPoint keyPoints = new MatOfKeyPoint();
detector.detect(ada, keyPoints);

// Draw circles on final image
Scalar color = new Scalar(127, 0, 255);
for (KeyPoint key: keyPoints.toList()) {
    Imgproc.circle(im, key.pt, (int) (key.size / 2.0f), color, 3/*Thickness*/);
}
...

You can look into dilation operation in opencv to enhance the edges.

https://docs.opencv.org/4.x/db/df6/tutorial_erosion_dilatation.html

Also look into this example below with Canny edge detection, approxPolyDP and minEnclosingCircle. It is very close to your question.

https://docs.opencv.org/4.x/da/d0c/tutorial_bounding_rects_circles.html

https://docs.opencv.org/4.x/d3/dc0/group__imgproc__shape.html#ga0012a5fdaea70b8a9970165d98722b4c

https://docs.opencv.org/4.x/d3/dc0/group__imgproc__shape.html#ga8ce13c24081bbc7151e9326f412190f1