T3 and T4 are both thyroid hormones. T3 is much more potent than T4. T4 actually converts to T3. So basically, simply put, for every 400mcg of T4, is 100mcg T3. The ratio is actually something along the 1:4.221When using T3, given your natural levels are typically between 15-30mcg naturally, you're able to use T3 up to 100mcg, and yes it's recommended to taper up and gradually taper down to adjust back to your own. T4 converts to T3. Yes, it raises your metabolism, body temperature, it helps with fat loss. Speeds up your thyroid function.
DNP is a totally different beast. Not sure how or why you're comparing the two. T3/T4 are hormones, DNP is an accidental discovery of chemicals that allowed people working around it, increase their body temperatures in dangerous levels. It's being used for quick fat loss but with many risks. DNP isn't something to play with if you're not willing to respect and educate yourself about the drug and its effects or even long term risks. You both sound as if you need to research more about it and are not ready to dabble in it without hurting yourself as the end result.