My just 4yo ds2 has suddenly turned into a bit of a monster!
I don't think that this kid of behaviour would warrant a trip to the GP or the thought that there is something wrong.
For me, I think it's the confidence that comes with starting school?
I loosely follow lots of parenting techniques, but nothing exclusive or 'proper' iykwim.
I have always counted down from 5, just to give them those extra seconds to comply. I want them to comply so give them the most time I think I can. My Dad said he used to count down from 10 because that's how long it took me to do as I was told!
The rule here is, I'll tell you twice, then I count down from 5, if they've still not done it, it's a time out.
Very rarely do I get to 1.
I've tweaked it a bot lately, because like you, I found I was saying no and telling them off all the time, so I have introduced a sticker chart.
I sat my boys (4 and 5) down and spoke them through it. I wrote 1-5 in big numbers across the top of the page and told them everytime I saw them do something really nice, they'd get a sticker and once they got 5 stickers, they'd get a reward, but we had to decide then what the reward would be.
I have a special bag that's filled with bubbles, whistles, bouncy balls, balloons, wind up toy, basically all the crap for a party bag and they said they wanted to choose something from there.
They did this, got 5 stickers and chose.
Then I wrote another 1-5 and they wanted this reward to be a small glass of pepsi. Done. Right now they are heading for a magazine from the shop.
At the bottom of the page, I drew a section off and this section is for strikes. If they get 3 strikes, they get a time out. If they argue about time out, they lose tv time, computer, football practice etc.
At the end of each day, I scrub the strikes out so we start a-fresh each day.
It seems to work (for now)
Just remember the mummy mantra, this is a phase, it too shall pass 