RT, I have two boys 5 1/2 and 2 1/2 both like to be independent, the youngest fiercely so to the point where I really struggle with him at times. He has to everything in his own way, in his own time and if I don't let him (which obviously I can't always) he has major tantrums, one example being his plate of food will get thrown or his drink too. He also loves anything to do with water and whenever possible I let him experiment e.g. playing with the hosepipe, washing my car.
My advice would be: a) don't try and force her to go the toilet when she wakes up, she is fighting you because she wants to be independent. Get her dressed and let her have an accident, or several if that's what it takes, she'll soon learn to go when she has to without delaying it. You have to let her do this especially at 4 years. And don't worry about this for school, they are prepared for accidents at school, you just send a spare set of clothes which she'll keep there if and when an accident happens. My oldest still regularly has an accident during the day, I don't worry too much about it, he's very lacking in bladder control and I know it's something he'll grow out of eventually.
b) don't give her a cup or glass at the table, or anywhere else. Give her water in a bottle which she won't be able to spill. Get her a princess one, or whatever she is into, so it's something she likes, but take it everywhere, play dates, restaurants etc so she doesn't have the opportunity to throw her drink. And then let her experiment with water at other times, give her a few cups in the bath. My two have empty bottles in the bath, they flood the bathroom and me most nights but they have enormous fun!
hehe - I'm sure you can tell that I've been there so many times over with my own nearly-4-year-old daughter!
In short, yes, a tougher stance on dangerous things, but also respect her growing understanding of the world and growing independence but explaining things to her - clearly and concisely - so that she knows why she must not do them. I do occasionally play on this a bit too - "do you think Big Girls do this?" - and big up the consequences sometimes too (DD slipped and cracked her head open, resulting in 2 stitches, which she remembers very well - it's what I call "incentive"!). I found having a newborn quite handy in being able to reinforce what Big Girls do as opposed to Babies - "babies spill water on the floor because they can't hold a cup properly, but Big Girls can hold cups properly, can't they?"
Also, if she does love water that much, maybe look to having a controlled opportunity for her to have water play in a safe environment - maybe a water table on the balcony/ outside or a tray on your kitchen counter with shapes and things. It's always about balance.
If it's something really potentially dangerous, then treat it as if she were about to run into the road or if she was playing around with boiling water - short, sharp stop, then a graphic explanation of the consequences and agreement that it's "not a good thing"... then an apology on her part if appropriate.
As for the loo, I'm guessing that your DD doesn't like being wet, but since she is in control of her own bladder, then I think you need to show (and tell her) that you trust her to go when she needs to. I also like to get DD to go before we leave the house, but sometimes she will just say she doesn't need to go, at which point I reply "ok, but we're going in the car for a long time and there are no toilets, so are you sure you can't squeeze a tiny wee out now?" Sometimes she'll tell me she really doesn't need to (and she'll be fine) and other times she'll smile coyly and go to the loo... Such is the way of a determined young lady!
the arrival of a sibling tends to affect the firstborn who often seeks attention in varying ways... so you're right there.
For me, the big decider on what to let slide or not is "is it dangerous" and "how much does it affect others". I would say that having spilt water on the floor, with a newborn is potentially dangerous, so that is why it needs to stop. Water is fun, no doubt about it, but there's a time and place for everything. Your Dd is old enough to grasp why some things need to happen and why some things shouldn't, so just explain to her why she mustn't. Tell her that it's dangerous and that if she slips or you slip, then you will hurt. Remind her of an incident where she fell over or slipped and cried (preferably lots!) to reinforce the cause-consequence thing. If she does it, then discipline and reinforce... and then get her to clean it up.
As for the potty - is she now old enough to decide for herself when to go to the toilet? Does she self-toilet? If so, then just give her that little bit of independence and let her take herself when she needs to (just make sure she washes her hands afterwards!). It's no great shakes if she doesn't go to the loo on demand, but rather better if she follows her own cues (which is what she'll be doing for the rest of her life). My DD is fiercely independent and often refuses to go when prompted, so I've stopped because a/ it doesn't really matter b/ I know she'll tell me when she needs to go and c/ there's be a very pointless long fight if I forced her, almost certainly ending in her still not going, but blood pressures raised all round!