TL;DR: get a 2x Barlow first - it's cheaper than an eyepiece and can be used with both of your 1.25" eyepieces.
If it works well and fiddling about with eyepieces and the Barlow in the dark gets a bit annoying, then look at buying a dedicated eyepiece.
A common wish when first looking at planets is to want to push the magnification as high as possible.
Many people, after getting their first telescope, have gone and bought an eyepiece which gives the maximum possible magnification, given the aperture of their telescope. Then, having tried it, they've come away disappointed with the blurry result they get.
This is because there are several factors conspiring against them:
The "resolving power" of the telescope. There are two formulae which give similar results: the Dawes Limit and the Rayleigh Limit.
The former can be calculated by dividing 116 by the aperture of the telescope in mm, and the latter by dividing 138 by the aperture in mm.
The result is the maximum resolution in arc seconds.
The quality of the "seeing" - how steady is the atmosphere and how much of it is there between you and the planet. If you're looking straight up then there's a lot less atmosphere getting in the way than if you're looking closer to the horizon. Also, if you're up on top of a mountain then you'll have better seeing than if you're down at sea level.
A good website which tells you what the seeing is like is meteoblue. This value is also in arc seconds.
The local conditions: concrete and tarmac will absorb heat from the sun throughout the day, and then release that heat at night. The heat will affect the air when you look through it, causing the view to shimmer.
So, to answer your question:
A good first eyepiece to get is not an eyepiece but a 2x Barlow lens. You can use this with your existing eyepieces and it'll have the effect of doubling their magnification.
Therefore, given you have eyepieces of 25mm and 10mm, adding a 2x Barlow will effectively give you two more eyepieces, of 12.5mm and 5mm.
After trying the 10mm eyepiece with 2x Barlow, you might find that the views are always crisp and never blurry, and you might want to try a 4mm.
(One trick with a Barlow is you can pull the eyepiece out a tiny bit to increase the magnification even further)
On the other hand, you might find that it's impossible to properly focus with that combination, and you'll need to ease off a bit on the magnification.
If that's the case, then (depending on the Barlow lens you buy) you might be able to unscrew the lens from the bottom of the Barlow and screw it into the bottom of your eyepiece, giving something like a 1.25x increase in magnification.
Otherwise you might want to buy an eyepiece of around 14mm, in order to use it with the 2x Barlow to get an effective 7mm.
Once you've played around a bit with the combinations and come to know the capabilities of your telescope a bit better, then you may want to buy a dedicated eyepiece of a focal length that you know will work well for you, with your telescope, at your viewing location, under normal viewing conditions.
The shortest focal length eyepiece that I have is only 3.3mm, and as such can only be used a handful of times per year - when all the factors coincide. It probably wasn't a very good use of my money. But when all the factors do coincide, it's given me the most amazing views of Jupiter and Saturn that I've ever seen!
