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The magnification is determined by the focal length (FL) of your telescope, divided by the focal length of your eyepiece.
So with your scope, it looks like about 900mm FL.. using a 3x barlow as well as a 4mm eyepiece!? That's crazy magnification, like over 900x magnification!
Firstly, that's way too much magnification for that scope.. it's way too much for practically ANY scope. The 4mm eyepiece on it's own would be too much magnification for the majority of nights you'll try and use it.
So let's step back to how you would "zoom". Start with a 25mm eyepiece. With your 900mm FL, that will give you 36x magnification. Saturn will be small, but at least you'll be able to centre it in the FOV of your eyepiece.
Then "zoom in" by choosing a shorter focal length eyepiece, maybe a 15mm. That will give you 83x magnification. Saturn is still a bit small.. so zoom in again. Practically, you'll probably want to go no more than 200x magnification. That means a 5mm eyepiece in your scope. Saturn will be bigger now, but it will also be dimmer and subject to the seeing conditions.. so it will probably be blurry and out of focus and not really what you were expecting to see.
On topic regarding "zoom eyepieces", they do exist. You turn the eyepiece the zoom in and zoom out.. however they don't retain focus, you'll need to adjust your focus after zooming in or out. Some brands are regarded quite well, such as the TV 3-6mm zoom (I think).. There's others that zoom over a larger range, like 10-25mm or so, but i'm not sure on the optical quality of those.
Without a doubt, some sacrifice has to be made in a zoom eyepiece - it's most likely the optical quality or ability to correct coma etc. Go to a star party or observing meet and try one out if you can, see if a zoom EP is really what you're after.
Hope i've helped.
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