Where do I input the information for line 4 of form 6251?

That is a computed amount. Look at the home mortgage deduction and go down to the last entry BEFORE Cash Contribution. You will find Certain home mortgage interest included above (6251)[Adjust] Here is what the help screen in Lacerte (Higher priced version of PTO) says;

Enter home mortgage interest that applies to Form 6251 if any of the home mortgage interest reported on Schedule A was not used to buy, build, or improve the main home (interest expense attributable to home equity debt).  An entry in this field carries to Form 6251 as a positive AMT adjustment.

NOTE: If the taxpayer is subject to excess mortgage limitations and you are completing the excess mortgage input section at the bottom of the Itemized Deductions screen, the program automatically determines the amount of interest attributable to home equity debt and carries the amount to Form 6251 automatically.  In this case, an entry in this field adjusts the program’s automatic calculation.  Refer to the “Excess Mortgage – Allocation of Interest and Points to Type of Debt and Forms/Schedules” worksheet.