It sounds like you want a detector to capably hunt for gold, and for coin/jewel/relic. An allrounder also means a detector that is capable of handling high junk areas, varying ground conditions, and beach work.
The GPX4000 can only be used for hard core gold detecting on mineralized ground, beach work-some beaches but not all, and lighly junked 'dig it all' coin/relic hunting, and only then in areas where interference allows you to work it.
The Explorer is at home at the beach, and in lightly junked areas, but useless on the goldfields, and acts like a pig anywhere else more often than not.
If you have the money for a GPX4000 and plan to use a detector a lot for gold, but need more from a detector than just that, then from a money and sense perspective, a new 2100v2 an XTerra70 (one of the best allrounders on the market) and a Musketeer plus every accessory known to mankind could be bought for the same money.
Makes sense to me, and you will not be leaving much, if anything, behind you for the GPX user.
I have only listed Minelabs for two reasons, firstly because you mentioned them, and secondly because they make the best detectors.