ENGINEERING ANSWER
The cited Rogers data sheet gives 4 mil RO4350B a process Dk of 3.33 ±0.05. This is a specific footnote to the process measurement; it is not permission to replace every design model with 3.33.
What to decide before release
Thin substrates make dimensional tolerance, copper thickness and etching more significant relative to the dielectric height. Ask for a field-solver model and a coupon built with the same foil and plating as the circuit.
Checks to put in your design brief
- Flag the 4 mil footnote in the material record
- Confirm the design model with the supplier
- Check manufacturable line widths
Important distinction
A generic 50-ohm width table for a thicker core can be badly misleading when scaled without accounting for copper.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.