Validation: Values from a list field column

Hi, I'm trying to pull information with rgpost for a validation function from a list field, but can't seem to get the right information from the array. Do you guys have an example anywhere for this?

When I do a var_dump of the field this is the output

array(12) {
	[0]=> string(27) " Home Equity Line of Credit"
	[1]=> string(5) "Cred1"
	[2]=> string(5) "$5.00"
	[3]=> string(2) "5%"
	[4]=> string(5) "$5.00"
	[5]=> string(0) ""
	[6]=> string(9) " Mortgage"
	[7]=> string(5) "Cred2"
	[8]=> string(5) "$6.00"
	[9]=> string(2) "6%"
	[10]=> string(5) "$6.00"
	[11]=> string(0) "" }

So I would like to add up all the values from the fourth column, but not sure how I can pull them from sometihng like that