What would I see with a telescope of 70mm aperture , 300mm focal length and maximum magnification of 150x
It's 70mm aperture, so the maximum useful magnification will be 165x.
If you have a 4mm eyepiece, you will have 75x magnification, and at that, you can expect to see rings of Saturn, moons of Jupiter in good conditions. Venus will show phases. I can't say much about Mercury.
You can see many surface features of the Moon. The view will be particularly good at 150x. You will have to use a 2x Barlow for that. Remember, the view will be good in good &quot;seeing&quot;.
If you are in dark sky away from light pollution, you can expect to see many globular clusters and nebulas particularly, the Orion Nebula.