What is Reddit's opinion of 1.25" 2.5mm/3.2mm/4mm/5mm/6mm/7mm/8mm/9mm 58 Degree TMB Planetary Eyepiece(Come with Original Box) (Black TMB 4 MM)?

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Given your two current eyepieces don't get you anywhere close to planetary magnification range, getting a 3-4mm eyepiece would definitely give you the best opportunity to see some details in your scope and get the most out of it. So in that sense it wouldn't necessarily be a waste, but since few telescopes have focal ratios as short as F/4, whatever eyepiece you buy for this scope would likely provide too much magnification for a future scope, so just keep in mind that this eyepiece would really just be to get the most out of this scope.

I would personally go with the 4mm from that link I posted above. It's the best value 4mm on the market that also isn't too hard to look through.

 

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This reply is just about the best overview explanation of what's out brand/quality wise.

The explicit answer to your question is going to partly depend on the specifics of you're scope, so you might reply back with that info. A fast focal ratio reflector is typically going to be more demanding of an eyepiece and expose the "flaws" of cheaper ones, and an f10 refractor or SCT will take just about whatever you put in it. Also, 4mm could be more magnification than useful depending on your scope as well.

That said, if you're looking for a budget 4mm in the $40 range I'd check out the TMB Planetary clones, like this.