Here is a list of all the items I take with me when trekking in mountainous regions.  When you go above 4000m it will be cold at any time of the year hence the inclusion of some very warm clothes.  I am going to use this list when I go back to Nepal in October.  I was previously there in January and added the items at the bottom of the list. You can download this list here and use it to help you organize what to take with you.  I have left some spaces so that you can add items if you wish.

Clothing for Trek
2 pairs trousers 1 softshell (warm), 1 lighter
2 T shirts
1 long sleeved shirt
1 midlayer zip Wool or fleece
1 down puffer jacket
1 waterproof jacket light weight shell
1 waterproof trousers Zip on over trekking trousers
2 beanies 1 light and 1 heavier that covers ears
1 gloves with inner gloves Inner gloves can be worn separately
1 buff Lots of uses
Trekking boots At least mid height
Trekking sandals (or similar) To wear in evening
3 inner socks Stops blisters
2 outer socks
underwear Quick drying
1 set thermals Top and bottom
Clothing for travel and days before trek
1 trousers Can be trekking trousers
2 shirts
Socks/ underwear
Walking shoes Runners or approach shoes
Toiletries In small containers
Sun glasses
1st aid kit In plastic bag
Leatherman Juice S2
2 1l Water bottles Or 2l bladder (hose may freeze)
Steripen plus back up purifying tablets
Head torch
Walking poles
Travel towel small
Sleeping bag and liner
Trekking wallet
Hand sanitizer
Spares kit Shoe laces, needle/thread, safety pins…
Spare zip lock bags 2 snack, 2 sandwich, 2 large
Camera, 2 batteries, charger, SD cards
Protein bars
Electrolyte tablets eg Shotz

All this fits into a combination of my 26l backpack (it is quite generous!) and a shared duffel.

For a winter trek I would also take;

another set of thermals
1 long sleeve shirt
Thicker puffer jacket Or soft shell jacket
Fleece neck warmer
2 warmer socks
Thicker sleeping bag liner

Please contact me if you want any more information about what to pack for your trek.  I always love to hear about your experiences and I wish you a great adventure.