-- ClientAddon.lua
------------------
local AIO = AIO or require("AIO")

if AIO.AddAddon() then
    return
end

-- rest of the code...
local button4 = CreateFrame("Button", "ButtonTest4", frame, "UIPanelButtontemplate")
button4:SetSize(150, 40)
button4:SetPoint("TOP", frame, "TOP", 0, -100)
button4:SetText("Learn |cff" .. string.format("%02x%02x%02x", classColor.r * 255, classColor.g * 255, classColor.b * 255) .. playerClass .. "|r spells!")
button4:SetNormalFontObject("GameFontNormal")
button4:SetScript("OnClick", function(btn) AIO.Handle("ButtonFunctions", "Button4Click") end)

-- Server.lua
------------------
local AIO = AIO or require("AIO")
local MyHandlers = AIO.AddHandlers("ButtonFunctions", {})
function MyHandlers.Button4Click(player)
    print(player:GetName()) -- prints in server console as we are on server side
end