I have a form that takes input of huge length of data equivalent to 4000 chars in both the fields, what would be the best way to insert them into a sfmc data extension ?
First you need to create the DE from email > subscribers > DE
When you create the text field in the DE, to hold this large text, keep the text field length as blank. This will keep the text field as VARCHAR.
You can only do this from email studio. You can't do this from Contact Builder.
When you create such field, you can insert any number of characters in that field in the DE.
If the question is submitting the form, you can write perhaps use SSJS but not an expert there...