Hi Jaime,
Select the dropdown field, open the 'Object' window, select the 'Binding'-palette, check 'Specify Item Values'.
Then, in the dropdown's change-event enter the following JavaScript-code:
if (this.boundItem(xfa.event.newText) < 4)
xfa.resolveNode("#subform[0].Textfield").presence = "visible";
else
xfa.resolveNode("#subform[0].Textfield").presence = "invisible";
You may want to replace '#subform[0]' with the name of the subform your textfield is located in and 'Textfield' with the name of the Textfield in question.
Regards,
Steve