They don't like you leaving your luggage with them if you are leaving the lounge, so this is not really the place to store your luggage if you are going to go and do some day trip flights the same day as your arrive and use the arrivals lounge. And in any case the arrivals lounge closes at 1400 so if you were to leave your bags there you would find that they were locked in and/or destroyed as abandoned luggage by the time you'd finished your day-tripping!

I think that your best bet, if I've understood your plans right, is to leave your luggage at the left luggage place. This is, conveniently, very close to the entrance to the T4 arrivals lounge. The cost is something like £5.50 per bag per 24 hours. The left luggage place is open until 2200 or thereabouts.