You are in certification mode. ! your mind is tuned to getting certified as PMP was rewarding. Its a good thing to get certified and learn new things. 

I would recommend that the next step would depend upon what your job demands , or it should align with your career aspirations

Ask yourself, out of the the 4 you have listed, 

1.  Which one is good for you in long term
2. Which area you want to make your career in?
3. Which of these would align most with your strengths

What stands our in a resume is SPECIALIZATION . If you do too many diverse certification and show multiple skills, you have less chance of getting hired. Clearly stand out as expert in one area.

All the best.

It seems like my activity is identical to yours. My employer is a smaller one but in any other year or so i am looking to step as much as a bigger firm just like Buy assignment online. I just commenced studying for the PMP but hadn't decided on assignment work yet.

Yes, the similar situation was with my Friend Who completed his PMP Certification in Chennai, He was looking for the next best course after his PMP then after he got to know about Six Sigma Course with suggestions of some institutes. He opted for Six Sigma Now he got good opportunities with both equal to PMP and Six Sigma So according to your interest and suggestions regarding your career you can opt for the courses you mentioned But As per my friends Experience you can once have a look at Six Sigma Good Luck....