Is she getting enough rest during the day and sleep at night? Even some 4 year olds need naps, or at the very least "quiet time" in bed to rest. I would also try putting her to bed 30 minutes to an hour earlier to see if additional night sleep helps.
Possibly she's feeling some jealousy towards her younger brother. Try to carve out a little time a few times a week to spend with just her. Let her know how special she is to you, and how much you enjoy your special time together, etc.
When she has a tantrum send her to her room immediately. Let her know that you aren't going to listen to her when she's acting like this and be consistent each time. A tantrum isn't fun if there's not an audience. If you're in the car and can't send her to her room you can tell her "I can't hear you when you speak that way" and pretend to not hear her and don't respond. If she continues send her to her room when you get home so she can cool off and remind her she's not to act that way when you're driving because you need to pay attention to the road to keep her safe. Your goal should be to put the responsibility of the tantrum on her, and to teach her to calm herself down on her own.
If she makes comments that you didn't do something for her like you did for your son you can always agree and say, "No, I didn't! You're a good listener!" or "I didn't know you liked fire trucks like your brother!" or "I didn't tell you you did a good job because you always do such a good job, and he's just learning!" and smile at her.
And if she seems to melt down more during idle time, keep her busy and enlist her as your helper. Have her fetch small items or toss things away when at home, "read" a book to her brother or herself when in the car, put stickers in a sticker book, etc.
Keep your cool, hopefully this is just an independent phase she's going through, or that more rest will help.