diff --git a/README.md b/README.md
index 2c48387..b524da0 100644
--- a/README.md
+++ b/README.md
@@ -9,3 +9,5 @@ Day | Name | Type of Algo
 1 | The Tyranny of the Rocket Equation | - Simple math problem
 2 | Program Alarm | - Intro to the crazy Intcode problems that are half the AoC days... <br> - Array (slice...) manipulation <br> - I used recursion
 3 | Crossed Wires | - Geometry kind of algo, finding intersections of lines on a grid
+4 | Secure Container | - May appear math-y, but it's really a string manipulation problem
+5 | 
\ No newline at end of file
diff --git a/day04/input.txt b/day04/input.txt
new file mode 100644
index 0000000..a5d5848
--- /dev/null
+++ b/day04/input.txt
@@ -0,0 +1 @@
+138307-654504
diff --git a/day04/main.go b/day04/main.go
deleted file mode 100644
index e8ac0ec..0000000
--- a/day04/main.go
+++ /dev/null
@@ -1,64 +0,0 @@
-package main
-
-import (
-	"fmt"
-	"strconv"
-)
-
-func numMatching(sliceToEnd []rune) int {
-	// fmt.Println(sliceToEnd)
-	// gets in a slice from a character to the end, returns an int of the matching characters
-	result := 1
-	for i := 1; i < len(sliceToEnd); i++ {
-		if sliceToEnd[i] == sliceToEnd[0] {
-			result += 1
-		} else {
-			break
-		}
-	}
-	// fmt.Println(result)
-	return result
-}
-
-func testNumber(num int) bool {
-	// cast to string to iterate through digits?
-	strNum := strconv.Itoa(num)
-	runesSlice := []rune(strNum)
-
-	duplicate := false
-
-	for i := 0; i < len(runesSlice)-1; {
-		if runesSlice[i] > runesSlice[i+1] {
-			return false
-		}
-
-		matches := numMatching(runesSlice[i:len(runesSlice)])
-		if matches == 2 {
-			duplicate = true
-			i++
-		} else if matches == 1 {
-			i++
-		} else if matches > 2 {
-			i += (matches - 1)
-		}
-	}
-
-	// if the entire for loop passes, return if there was a duplicate or not
-	return duplicate
-}
-
-func main() {
-	start, end := 138307, 654504
-	possibleCombinations := 0
-
-	for i := start; i <= end; i++ {
-		if testNumber(i) {
-			possibleCombinations++
-		}
-	}
-
-	fmt.Println(possibleCombinations)
-	// fmt.Println(testNumber(111122))
-	// fmt.Println(testNumber(123444))
-	// fmt.Println(testNumber(112233))
-}
diff --git a/day04/part1/main.go b/day04/part1/main.go
new file mode 100644
index 0000000..6116614
--- /dev/null
+++ b/day04/part1/main.go
@@ -0,0 +1,47 @@
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	start, end := 138307, 654504
+	possibleCombinations := 0
+
+	for i := start; i <= end; i++ {
+		digits := makeDigitsSlice(i)
+		if isIncreasing(digits) && hasDuplicate(digits) {
+			possibleCombinations++
+		}
+	}
+
+	fmt.Println(possibleCombinations)
+}
+
+func makeDigitsSlice(num int) []int {
+	result := make([]int, 6)
+	for i := 5; num > 0; i-- {
+		result[i] = num % 10
+		num -= num % 10
+		num /= 10
+	}
+	return result
+}
+
+func isIncreasing(digits []int) bool {
+	for i := 1; i < len(digits); i++ {
+		if digits[i] < digits[i-1] {
+			return false
+		}
+	}
+	return true
+}
+
+func hasDuplicate(digits []int) bool {
+	for i := 1; i < len(digits); i++ {
+		if digits[i-1] == digits[i] {
+			return true
+		}
+	}
+	return false
+}
diff --git a/day04/part2/main.go b/day04/part2/main.go
new file mode 100644
index 0000000..01883ca
--- /dev/null
+++ b/day04/part2/main.go
@@ -0,0 +1,80 @@
+/*
+Only change from part1 is adding a decorator to the digit slices to shrink large
+groups (more than 2 digits) into a single digit. Then pass that into the same
+checks for a duplicate & all decreasing
+*/
+
+package main
+
+import "fmt"
+
+func main() {
+	start, end := 138307, 654504
+	possibleCombinations := 0
+
+	for i := start; i <= end; i++ {
+		digits := shrinkLargeGroups(makeDigitsSlice(i))
+		if isIncreasing(digits) && hasDuplicate(digits) {
+			possibleCombinations++
+		}
+	}
+
+	fmt.Println(possibleCombinations)
+}
+
+func makeDigitsSlice(num int) []int {
+	result := make([]int, 6)
+	for i := 5; num > 0; i-- {
+		result[i] = num % 10
+		num -= num % 10
+		num /= 10
+	}
+	return result
+}
+
+/*
+shrinkLargeGroups will return a new slice with any groups larger than 2 shrinked
+down to 1. e.g. 111223 -> 1223
+*/
+func shrinkLargeGroups(digits []int) []int {
+	// from start of number & ensure i+2 is within bounds of the digits slice
+	for i := 0; i+2 < len(digits); i++ {
+		if digits[i] == digits[i+1] && digits[i] == digits[i+2] {
+			// figure out how many items to remove
+			removeUpTo := i + 1
+			for removeUpTo < len(digits) && digits[i] == digits[removeUpTo] {
+				removeUpTo++
+			}
+
+			// copy the values into a new slice
+			newSli := make([]int, 0)
+			for j := 0; j <= i; j++ {
+				newSli = append(newSli, digits[j])
+			}
+			for removeUpTo < len(digits) {
+				newSli = append(newSli, digits[removeUpTo])
+				removeUpTo++
+			}
+			digits = newSli
+		}
+	}
+	return digits
+}
+
+func isIncreasing(digits []int) bool {
+	for i := 1; i < len(digits); i++ {
+		if digits[i] < digits[i-1] {
+			return false
+		}
+	}
+	return true
+}
+
+func hasDuplicate(digits []int) bool {
+	for i := 1; i < len(digits); i++ {
+		if digits[i-1] == digits[i] {
+			return true
+		}
+	}
+	return false
+}