I left White Orchard at level 4. I had done everything there to do, found all the poi's, did all the quests except one that bugged out on me, it kept telling me to read a letter, which I did. I'm in Velen now, still level 4, but all the quests I find are level 7-33, if I travel I run into bandits that are level 10-16.

I came across a monster, but it was level 14, and it killed me in one hit. Where am I supposed to be exploring in Velen that is a suitable level for me?

It's because you found the treasure before the note. Same here; doon't worry. I was exactly in the same position. I'm dead many times when I tried to do some "?" before the main quest.

So what I did is following the main quest and find hendrick's village.
Open your quests book and follow them

You can find some do-able quest at the N-E of the map near the "bridge".
And at the West mostly, where is hendrick quest.

Then, I stopped. I was in front of the Baron castle.

stick with the main story for a bit to boost your level

When you get to Velen do the main story for a good while before exploring. While there are some side quests and things you can find to do, most of it is going to kick your ass. Your time is better spent doing main quests that are tailored for your level first.

I'm actually finding the opposite problem. I want to do all quests and I'm doing them in the order of lowest level to highest, but I already find my level is starting to get above the recommended levels. I guess it's my fault for being a completionist. I figure when my level gets high enough above the recommended level, I'll have to up the difficulty to the highest setting (currently second highest).

Not that I'm complaining! I like that enemies don't scale. That's as it should be and I thank CDPR for taking the risk of doing it.

With doing all the quests you leave Orchard at around level 4. How the hell are you killing level 10+ group of enemies when you are on level 4 in Valen?

What's the secret oh master witcher?

I don't know what difficulty everyone's at, but on normal my level 5 Geralt can take on groups of 7-10s easy cheesy. If I see a quest a few levels higher it's almost never a problem.

I'm currently level 8, doing quests recommended as level 6. However, I've killed level 10 bandits as far back as when I was level 5. It's not easy, but do-able on the second hardest difficulty. I think lvl 10 is the highest enemy I've killed at this point.

My point wasn't about enemies in the surrounding area. It's about missions and getting too far advanced for them.

I'm killing level ten bandits, but I have to use Thunderbolt, Swallow, and I'm dodging and diving around like mad trying to spread them out a bit. Signs do absolutely no good against them, so I am parrying a lot as well to open them up. Though blocking against heavy weapons also seems to be useless, they just break right through.

I've taken the advice given and I'm focusing on the main quests. Finally bumped up to level 5.

I'm lv 4 and just spent the night roaming the fields, going after ? spots on my map. Most cases I run into stuff Lv 8-12 and don't have too much trouble (normal difficulty). I find a large part of my success is simply from doing the best crafting I can for my level and using guerrilla tactics. Fight smarter, not harder I guess..

My best advice is to hit up some missions because they boost your xp way faster than just doing random treks into dangerous territory.

Yeah i'm having the same issue, i'm level 5 and everything is way above my level.
I'm ok when it comes to some nekkers, bears or bandits that are around level 9-10 but i met a wyvern and a basilisc (or how it's called in english) that were level 14 or 15. I may be able to kill them but that would ruin the immersion of the game, doing such monsters so early (they also probably are related to some quest). I'll probably stick to the baron main mission since i already have to start it.

I was killing lvl 10 bandit groups like nothing as soon as I arrived in Valen too.
Heck, I killed the strongest monsters (minus the griffin) in White Orchard at lvl 1.
And I'm on Deathmarch.

Use Quen and roll. Hit them once or twice and roll away. Don't hit twice the guys with hatchets because they will stagger you or blow up your Quen.
Always remember to kill the archers first.

Invest points in Axii. Use it on bandits, especially ones with shields or 2 handed axes or pikes - with Axii, they'll lower their shields and weapons where you can go in for the kill. Works also on some monsters to stun or stagger, giving you a chance for clear, solid hits. You will also be able to influence people in dialogue. Very helpful.

On a related note, what are the levels of the end game main quests and what is the highest level quest seen in the game?

Stop being a cry baby. You are a Witcher god damn it! I took on 1 Wyverns, marked red, while being clearly overlevelled! Took me 40 minutes but I managed to kill them :P And honestly, follow the main quest for the time being. Later on you will be on equal grounds.