Hi! I would definitely NOT try to re-introduce Juno to your working trio! Rather consider keeping him alongside the others with interaction through the bars or eventually getting him his own, ideally character compatible mate.
If you can get to Windhwistle Warren near Cheltenham as your closing experienced guinea pig rescue that offers boar dating at the rescue, you can avoid ending up with another dysfunctional boar bond as you come home with a new companion only if they click and always have the rescue to fall back on if things go wrong at some point. The Blue Cross at Burford is another good rescue in your area, but they do not offer dating for single guinea pigs.
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The only working and stable quartets I know of are elderly boars whose testosterone has mostly fizzled out or groups that consist of disabled and carer companions and that have completely different dynamics to normal boar groups.
Boar trios are great if they work out, but the vast majority baby trios end up with a separation and even a mix of ages can be very difficult to balance out personality-wise. If you have a functional trio, I would not risk upsetting the apple cart under any circumstances; if you are unlucky, you can end up with three or even four single boars that won't get on with any of the others!
