Crack this safe

Yet another investigation calls you to the home of John Speldnick, a criminal with ties to a terrorist organization that had plans of bombing the subway station in the metropolitan area. Luckily, your team has thwarted their attempts of attack, but there is very sensitive information hidden inside of the safe in the man's house. The safe is very heavily fortified, meaning that fire and other forms of brute force used against it will be ineffective. The only way to get into the safe is to crack the 4 character passcode, made of letters and numbers. The only clues you have are the images left near the safe. Use them to crack the code and secure the sensitive documents. Good luck.

Comment for hints

Character 3:

is represented by a pH graph. The neutral value on the pH scale is 7.

Character 1:

is represented by a graph of a vertical line, alongside a photo of a mountain. The mountain implies the gradient of the graph is relevant. The gradient of a vertical line is undefined.

Character 2:

is represented by a stock graph falling over time. This depicts loss, the opposite of profit. There are four panels in the well-known "Loss" comic.

Character 4:

is represented by a collection of pictures of technologies. Two vehicles: a plane and a car. Two electrical devices: a lightbulb and a Tesla coil. A strange sketch depicting odd numbers, a visible 1 numeral, and a Share button, aside an iPhone 5.

Tentative guess:

The code is 0 4 7 5

Reasoning:

I'm unsure on the exact format of C1, and unsure of my line of thinking on C2 and C4. Additionally, since the question asked for a four character code made of letters and numbers, it's difficult to know if that's a red herring or not.

• Hello! Welcome to the Puzzling Stack Exchange (Puzzling.SE)! Since you are new not only to this site, but to the entire Stack Exchange community, I strongly suggest you visit the Help Center; but apart from that, nice answer! Happy Puzzling! :D – Mr Pie Sep 30 '18 at 4:47
• Why was this accepted? – Archmage Oct 23 '18 at 23:11
• I don't know... it is most likely, however, that this is the correct answer; though I am not the person to ask, as I did not present this puzzle :P – Mr Pie Oct 24 '18 at 10:38

Attempt at a partial answer, I am not sure if it is correct or, if it is, where to go to from here.

First image (labeled 3):

The letter "B"

Explanation:

It appears to be a pH scale of Lemon (pH 2), Coke (pH 2.5), water (pH 7, typically), toothpaste (varies depending on content), bee (sting, pH 5.0-5.5). If it is a scale than the bee (B) is the odd one out. Alternatively it could simply be "H", taken from pH = potential Hydrogen.

Second image (labeled 1):

The letter "m"

Explanation:

Both the graph and the hill can be described by their gradient. In maths the letter "m" typically represents the gradient (i.e. y = mx + c).

Forth image (labeled 4):

Not sure, but could be the letter "i"

Explanation:

Several electrical devices are shown. The flow of current is represented by the letter "i", there is also an iPhone.

Since

each character is either a letter or a number,

there are

26 + 10 possibilities for each character. Over four characters that's 1,679,616 possible combinations.

Assuming you can

input a new combination every ten seconds,

it would take roughly

5.326 years to try every possible combination. Paying low-salary police officers $12 an hour around the clock for 5.326 years costs roughly 406,480 dollars and 32 cents. Depending upon how time-sensitive the information is, I can crack the safe for you given only 5 years and$400,000.