There are many ways to do this in Baan IV (Even more in Baan V!)
If you wish to make a field display only for your session only for all users / all instances, you can modify the form, and change the form field option 'Field Type' to display. Post back if you need additional instructions on that.
If you wish to make display only for all sessions, particular users, you can make entries into the session Maintain Table Field Authorizations (ttaad4133m000) and give the users 'Read' privelages.
If you need to make it display only based on conditions at run time, you need to modify the script, and set the 'attr.input' variable to true or false in the before input section of that field. Something like:
if your.condition then
attr.input = false |* No entry can be made
attr.input = true
endif |* Entry can be made
I'm sure there are other ways as well. Post back if none of those will work in your scenario.