Hi Deeneil,
If you bought a 2x barlow and used it with your 4mm ep, you'd have a mag of 300x, which you may be able to use on nights of very good seeing. Other nights you would have a wobbly image, like a mirage. A 3x Barlow with your 10 mm ep would give you about 200x - you'd use it more often, but it would not be very different to using your 4mm by itself.
The best way to improve your observing results is practice. you will see more and more as you look. Having someone help you will make a huge difference. It might be worth putting a post in the beginners forum to see if there is someone near you that you can do some observing with.
Jupiter would definitely be worth a look as a starting point - it is the brightest thing in the night sky at the moment and you should be able to see its moons and some cloud bands etc.
Regarding equipment and observing, you may find the following link helpful.
http://www.skyandtelescope.com/equipment/basics
Don't give up, take time and you will see more and more, but some of it will seem very subtle at first, not like what you see in astroimages.
All the best!