I worried through my first 12 weeks in my first pregnancy as didn't have any symptoms and I am worrying now lol
200mg a day of caffeine is the recommended.
The best decaf Tea I have found is PJ tips, it really good.
Its up to you what you do thought. I think a cup of tea is about 50mg (so it says on the coca cola website lol) so you can have 4, but then think about the other caffeine you have, like the diet coke etc.