It sounds really tough but I can also tell how much you love your kids. I believe you can do this!
Some PPs have been very judgemental and superior without actually being able to offer any advice. You'd think the perfect parents would have more tips than the rest of us put together!
I think you need to keep it very simple for a start. At 3 and 4 they just haven't reached a point in their cognitive development where they can process lengthy explanations or instructions. They also won't understand delayed consequences/rewards either (e.g. if you're naughty now, we won't go to the park later/tomorrow), so you need to keep consequences instantaneous as much as possible. For example, if they are already at the park and they start misbehaving I'd give one clear, short warning (delivered in a sharp tone of voice) and if the behaviour persisted, I'd take them home straight away. If you're at home and they are throwing things, take everything off them.
Remember that attention is a reward. So if you pay them more attention when they're naughty than you do the rest of the time (easily done, because when they're being good we all tend to leave them to it!), you're effectively rewarding the bad behaviour. So make sure to give loads and loads of verbal praise when they are being good, and when they are behaving badly you need to remove whatever object they are throwing/remove them from the playground/take away the food they are chucking about etc with minimal interaction. So no big fireworks or speeches; just a firm "no" and then deal with situation without further interaction.
On the same note RE attention, make sure you spend a good 15 mins each day just completely focused on the kids, playing with them and with no screens. The more positive attention you can give them the better.
Without making a thing of it, I'd also scale right back with treats, baking, trips to the park etc and add them back in as rewards for good behaviour.
Good luck!