You're probably just not used to thinking about what you have to pack versus what would nice to have. When you go for longer, you can take more in hold luggage and paring back isn't as strict. For carry on only, you have to change your mindset to "What do I NEED" rather than "What would be nice to take?" If you can do without it for a few days, it's not necessary.
I did a five day trip to Italy in May, and had the same weather concerns. In your position, clothes-wise I would take:
4 pairs of knickers plus spares
Couple of bras and a bralet
4 pairs trainer socks
Trainers*
1 pair comfy sandals
2 pairs shorts
2 team tops
1 spare t-shirt/top (plus a t-shirt for the flight*)
Pair of jeans or trousers*
1 jersey dress
A cardigan*
A light jacket*
A vest/t-shirt and shorts sleepset
The asterisk items you wear on the plane. They won't get sweaty so can be worn again. You could take a layer off if you're too warm on the flight. Wear either the shorts or jeans and team top for the race days, changing into the dress and sandals for evening. Add cardigan or light jacket if it's cooler. On non-race days, the t-shirt plus jeans/trousers you wore on the flight. If it's sunny and warm, the dress and sandals combo. Pick items that generally go with each other colour-wise, plain rather than patterned is easier.
That should leave plenty room in your cabin case and your backpack/additional bag for toiletries and chargers etc. I don't have period stuff to contend with but I have contact lenses and need space for saline/case/glasses, I can't do without those. I can do without make-up and perfume (I usually smell of Autan or Jungle Formula, perfumes attract mosquitos) so don't pack those, can't do without Sensodyne toothpaste so can't make do with any old Colgate travel tube. Like a PP says, you have to compromise where you can to allow for where you can't.
We tend to go on holidays of 18-21 days so I'm always planning on doing a couple of washes through the holiday, and for that length of time I do take tweezers and cotton buds and shizz like that. I definitely wouldn't for four days though!
Hope the hospital appointment goes well...