I have two paragraph of 10pt texts in my postcard design. One on white, and one on an image. Both text require using brown color. Currently the values is at C42 M65 Y96 K41, I am worry that the result might be blurred text if the plates are not registered correctly. What is right thing to do in this situation?
As @Scott says, 4 color small text isn't recommended.
Perfect alignment of the inks isn't possible in real life. Even the best printer using the best equipment can't prevent the paper from expanding during print. The paper gets wet and in the worst case scenario the paper can grow by maybe 1-2 mm!
A way to minimize the problem is to see if it's possible to create a similar CMYK color using 3 inks instead of 4.
Most browns containing both cyan and black can actually be reproduced without the cyan. So you can set C=0 and nudge the other percentages until you find a balance which looks similar:
You do not say which CMYK profile you are using. It's important that you find out which profile your print shop recommends.
In this example I have just chosen a widely used profile: Coated FOGRA39.