If it's sound quality that matters to you the most, the only two dongles that can be closer to Plus4 performance are FiiO KA17 and iBasso DC04 Pro. And I would have to listen to them to believe it. Plus4 is not in the same league of most sub 200€ dongles, including tp50. I don't feel I lose much compared to Topping DX5 that costs 3x as much. So if you want an upgrade and you like FiiO, get KA17. DC04 Pro should be very close as well. If you want to take the risk on Plus4, you won't be disappointed. Outperforms BTR15 in pretty much every aspect and you'll notice that in anything you listen to. The least expensive of the three seems to be DC04 Pro at 120€ and you can buy it from iBasso EU with 1-3 day delivery. Next Plus4 at 140€ and then KA17 at 159€. At 140€ (Plus4) add 23% IVA here in Portugal, so it goes up to 170€.
Looking closely now at KA17 specs and another review, it's probably as good as Plus4. It's probably the one I will like to try as soon as I can. If the THX amp affects sound quality, it's a serious competitor.