like can i use the items and rules?
...
No. They are entirely different games.
The only thing that you could use from a 4.0 starter kit is the little figurines (If they came with the kit). The rules, classes, items, monsters, etc. are completely unusable in 3.5. This is one of the reasons why I dislike 4.0. In previous versions there was always some way to use stuff from earlier or later versions. But 4.0 is just too different.
Key differences between 3.5/4.0:
1. 3.5 has no powers. 4.0 combat is centered entirely around powers.
2. 4.0 dispensed with the classic spell system, replacing it with certain powers and rituals. These cannot be interchanged.
3. 4.0 monsters use powers as well and thus are not balanced for 3.5 characters
4. In general, the characters are different. 4.0 characters have "Healing Surges" which allow them to regain some lost HP. 4.0 characters have a shorter skill list and don't get more skill points.
5. All sorts of other rules minutiae. For example, dying in 3.5 and 4.0 are different. In 3.5 you don't die till -10 but you're disabled at 0 and unconscious at -1. In 4.0 you can't go negative. Once you're at 0 you're unconscious and have to make saves each turn to see if you get closer to dying. Spells no longer provoke attacks of opportunity in 4.0. New 4.0 feats are incompatible with 3.5 feats and are generally more bland.
6. In 4.0 you can't multiclass as you could in 3.5. Instead of prestige classes, there are "Paragon Paths" that branch off of your main class once you get to a certain level. If you want to combine classes in 4.0, you need to make a "Hybrid" class combining elements of the two. Rules for this are detailed in PHB II 4e I think.
And many more things.
Poll