How do I detect what pattern buttons were pressed?

b3wz's idea is good, this is mine:

local Player = game.Players.LocalPlayer
local Door = game.Workspace.Door
local Parts = {
["Part1"] = 1;  
["Part2"] = 2;
["Part3"] = 3;  
}
local Part1 = game.Workspace.Part1
local Part2 = game.Workspace.Part2
local Part3 = game.Workspace.Part3
local num = 0

Part1.ClickDetector.MouseClick:Connect(function()
    local target = num+1
    if Parts["Part1"] == target then
        num = num+1
    else
        num = 0
    end 
end)

Part2.ClickDetector.MouseClick:Connect(function()
    local target = num+1
    if Parts["Part2"] == target then
        num = num+1
    else
        num = 0
    end 
end)

Part3.ClickDetector.MouseClick:Connect(function()
    local target = num+1
    if Parts["Part3"] == target then
        num = 0
        Door.Transparency = 1
        Door.CanCollide = false
    else
        num = 0
    end 
end)

This is an idea on how you can do it:

local Pressed = {}

-- when you listen to buttons being pressed do something like this maybe
if #Pressed > 4 then
    table.insert(Pressed, button.Name)
else
    if Pressed[1] == "Button1" and Pressed[2] == "Button2" and Pressed[3] == "Button3" and Pressed[4] == "Button4" then
        -- make part transparent
    end
    Pressed = {}
end

Might not be the best way to do it but was the first idea that came to my head