Why am I only level 4 after leaving White Orchard?

Apparently there is a slightly reduced level of XP gain (90%) for Blood and Broken Bones.

Although at just into level 4, you would stay there even at 100% gain.

I don't think level 4 is that low for leaving White Orchard

I suspect the level 8 comment on reddit is just an anomaly. Certainly others in that thread and other threads suggest 4/5 is normal.

There's definitely a jump in difficulty from White Orchard to Velen (which also has a much bigger spread of enemy levels) which is probably much more noticeable on a harder difficulty setting.

My last play-through I was on Normal difficulty and I think I left around level 5, but then I did go back to the wolf area in the south 5 or 6 times to kill a load of wolves (along with a few drowners and nekkers that weren't necessary to kill) for the money. I remember I was certainly quite delicate trudging around Velen (at first), and there were certainly a lot of areas I had to avoid altogether until I got some easy quest XP under my belt.

The level 11 requirement for the Griffin gear seemed so far away!

Addendum: I've just played through this on Death March diffficulty, and after doing all quests in White Orchard, I left the area having only just levelled up to level 3

There is nothing unusual about this. I just checked my saved game: I was level 4 (Exp 170/1000) when about to enter the elven ruin in velen with Keira Metz. I have no mods or unofficial patches installed and my game is on version 1.31, and I had played on Death march.

The place you start in velen is not good as you are surrounded by high level points of interest. Ignore them (run away) and focus on the quests, they are easier and should help to level up.