Take a look a directory, where B4i is installed (typically, C:\Program Files (x86)\Anywhere Software\B4i.
Inside a subdirectory Libraries you will find XUI Views.b4xlib. Copy this file somewhere outside, rename to .zip and extract B4xFloatTextField.bas
Rename B4xFloatTextField.bas to (for example) B4xFloatTextField2.bas
Create a new project and add B4xFloatTextField2.bas class. In Designer and in code use B4xFloatTextField2 instead of B4xFloatTextField.
Then search "HintFont" inside B4xFloatTextField2.bas and Log (HintFont) inside Public Sub Update subroutine, Theoretically it's possible to change HintFont and to call Update in own module, but it's not very comfortable