WowAce

It's real easy to do:
RepXPBar.lua - Add this code:
-- register module
local PetXPBarMod = Bartender4:NewModule("PetXPBar")

-- create prototype information
local PetXPBar = setmetatable({}, {__index = Bar})

function PetXPBarMod:OnInitialize()
    self.db = Bartender4.db:RegisterNamespace("PetXPBar", defaults)
    self:SetEnabledState(self.db.profile.enabled)
end

function PetXPBarMod:OnEnable()
    if not self.bar then
        self.bar = setmetatable(Bartender4.Bar:Create("PetXP", self.db.profile, L["PetXP Bar"]), {__index = PetXPBar})
        self.bar.content= PetPaperDollFrameExpBar

        self.bar.content:SetParent(self.bar)
        self.bar.content:Show()
        self.bar.content:SetFrameLevel(self.bar:GetFrameLevel() + 1)

        -- TODO: real start position
        self.bar:SetPoint("CENTER")
    end
    self.bar:Enable()
    self:ToggleOptions()
    self.bar:ApplyConfig(self.db.profile)
end

PetXPBarMod.ApplyConfig = RepBarMod.ApplyConfig
PetXPBar.ApplyConfig = RepBar.ApplyConfig
PetXPBar.PerformLayout = RepBar.PerformLayout

In Options directory:
RepXPBar.lua - Add this code:
local PetXPBarMod = Bartender4:GetModule("PetXPBar")

function PetXPBarMod:SetupOptions()
    if not self.options then
        self.optionobject = Bar:GetOptionObject()
        local enabled = {
            type = "toggle",
            order = 1,
            name = L["Enabled"],
            desc = L["Enable the PetXP Bar"],
            get = function() return self.db.profile.enabled end,
            set = "ToggleModule",
            handler = self,
        }
        self.optionobject:AddElement("general", "enabled", enabled)

        self.disabledoptions = {
            general = {
                type = "group",
                name = L["General Settings"],
                cmdInline = true,
                order = 1,
                args = {
                    enabled = enabled,
                }
            }
        }
        self.options = {
            order = 101,
            type = "group",
            name = L["PetXP Bar"],
            desc = L["Configure the PetXP Bar"],
            childGroups = "tab",
        }
        Bartender4:RegisterBarOptions("PetXP", self.options)
    end
    self.options.args = self:IsEnabled() and self.optionobject.table or self.disabledoptions
end

In locale directory:
enUS.lua - Add entries:
L["Configure the PetXP Bar"] = true
L["Enable the PetXP Bar"] = true
L["PetXP Bar"] = true

--
Hodor, Draka.

Is the Pet XP Bar something thats usually displayed in the BottomBar Cluster? I don't think it is, thats why BT4 won't touch it.

Ahh, I see. Guess I'll patch it in on my own then (and again after every release.)