What would I be able to see with 70mm aperture telescope with 300mm focal length with maximum magnification of 150 x?
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 "seeing".
If you are in dark sky away from light pollution, you can expect to see many globular clusters and nebulas particularly, the Orion Nebula.