Hi Kimmy
The equipment that you currently have is more than competent to give you good results and I would speculate, if you are not happy with your pictures, that it is either your detail knowledge of the equipment and/or your photographic technique rather than your equipment that requires a review. Throwing some time, energy and effort at learning your equipment and improving your technique is far more likely to provide a better return than simply throwing additional money at equipment. Only you are likely to know when you have reached the limit of your existing equipment, and when an upgrade is required. Exploit your equipment to the limits, understand what the limits on your equipment actually are, and only when you have reached those limits, consider upgrading your equipment.
While the above comments may seem to be presumptuous, it has become too easy for people to suggest improvements to equipment, and in some cases, such improvements may make a difference. But if you are not exploiting the equipment you currently have to its fullest extent owing to a lack of knowledge and/or questionable techniques, focusing on each of those aspects is likely to improve your images far more than any additional, expensive equipment.
OK, got that bit off my chest. So when you are comfortable with your existing equipment and technique, and you feel you have reached the limits of your existing lens, I suggest that you look at the following two lenses for your stated needs:
- 35mm F1.8G DX; relatively inexpensive and a reasonably fast "normal" prime lens. This should allow for your portrait-type pictures of dogs from around 4 metres away, can work well in relatively low light without a flash, and will double up as a "normal", walk-about lens. Further, a single step towards or away from the subject, when at 4 metres, can make a substantial difference to your framing of the image.
- your question regarding the new 70-200 f4 lens is apt but only you can answer for your needs (and desires and budget!). All accounts indicate that this is a great lens and will work well in your desire for pictures of fast-moving subjects at around 30m. The 70-200 f2.8 may be a faster lens, but it is also heavier and more expensive. If on a limited budget, also consider the 70-300 VR, especially if your photography is almost exclusively undertaken in good light. At a third of the price of the 70-200 f4, it is not limited to a third of the ability. Sure, it does have limitations, but few that cannot be overcome with equipment knowledge, technique, and reasonable light.
Take care....
Scott