What is a good setting for 10 mil stencil with 4mm heigh lettering

Welcome to the forum.

The search function here provides years of shared information. Here is an old thread which may be where you got the settings of 250/25. Mylar 10 Mil stencils - #3 by ptodd

Since materials are variable, it could be that you will need to adjust your settings and test until you get a good result. If you believe items are melting, increase your speed. There are some very good templates for testing settings in the Free Laser Design section.