Just got home (Bangkok) exactly at this moment, and I'm a bit too exhausted to make a full description. But in case... the first 50-60km after you enter Highway 4, coming from LP, were ABSOLUTELY HORRIBLE, very harsh conditions, at times, I thought the truck wound't go through even with 4x4 and going in the right "lanes". Didn't matter much where I was pointing the steering, the "thing" had a life of its own. Didn't help either the rain, not very strong, but strong enough to make a lot, but A LOT of mud.
After that, the rain stopped and the road was actually quite nice, some potholes, a few "patches" of asphalt (not more than 1-2km long, 2 or 3 of them), enough to make 90-110km/h at some points, that until Xayaboury. Once we got there, it looked like a city once again, with a few good km of good asphalt, some restaurants, shops, gas stations. About 5km after the city, again some good dirty roads, a few potholes, nothing major...
then "hell" started, I don't remember exactly when, but there were no roads, just a few centimeters of straight dirty road in the middle of hundreds, THOUSANDS of potholes, some of them 40-50cm deep... the car was shaking like hell, one of the metals I have under it to protect must have gotten lose, and I can still hear it "banging" down there, must have that checked soon... that lasted for, I don't know, 40km or so, really not "the best" (and near of "the worst"). Then once again a good 30-40km of good roads, and "second hell" started, about 30km of flat roads covered in red mud, near thousands of rice fields, and very strong rain, many times the front wheels fell in a big pothole and the spray of red mud covered the windshield for like 2-3s, even with the wiper at maximum, it was hard to see the direction I was going... luckly, not that many people around... :O
Anyway, that eventually ended, and I was back to the dry-roads-with-huge-potholes, until I got to the beginning of a "forest" area, no more rice fields or anything, just forest and mountains around me... and a lot of construction going on there, the road was... non-existent, just deep mud, the car sliding sideways, no matter the speed, but the layer of mud was easily 50cm high, getting to my door almost (I have some pictures and videos of the way, haven't seen them though), at that point I was just going along with the adventure 1- I was 250km away from Luang Prabang already; 2- I was following another Hilux which seemed to know the way. I eventually passed that point, after almost going to the ditch on the side of the road a few times...
I was expecting to run the 313km I managed to sum from the GT-Rider Lao map I have, but it ended up being more, about 385km until the Friendship bridge. The 313km I was counting actually ended in Pak Lai, the road was already very good at that point, and for the last 100km or so, it was mostly really, really good roads, those small rocks, no mud, and some areas with a kind of "thick stone asphalt" and straight ways, enough for 120-130km/h (maybe more), no one around, nice views, nice curves... I was just a bit disappointed I had to keep going, from Pak Lai, many many km more than I was hoping.
Ah, all the small rivers have bridges on them, most of them new bridges, and the largest one (that was not the Mekhong), they are making a new bridge to replace the wood/metal one. The only major delay I had was to cross the Mekhong, 20min for the ferry to arrive, 30min waiting on the ferry, and 10min to cross it (really fast running river, the ferry had to go like a crap, "walking sideways"). They charged me 35k + 5k Kip for the crossing.
Last for now, about the Friendship bridge... everything went smooth over there, the Lao side has a brand new very big "Portal" for immigration/customs, much bigger than the Thai side (actually, about the size of the one in Nong Khay/Vientiaine), the just checked and stamped the papers for the car, the passports... and I askef the g/f to see with the guy what's up with the "Visa Upon Arrival", he said it's ok, they can make it there (at least he confirmed he could issue a Visa for my brazilian passport, which is a proper sticker/visa, same as Cambodia), he actually confirmed it twice "mi miiii!!".
On the Thai side, no one was there, I just passed with the car, parked on the side of the immigration office, went there to the cabin, they checked the documents, stamp here, stamp there... and they asked what was the car, the g/f said it was the SportRider and the guy "oh, that thing covered in mud was a SportRider? ok, ok!", and... that was it.
BTW, the small roads in Loei are not so much better than the ones in Laos...
I'll rest now, really exhausted, once I manage to put my brain back in order, I'll come back here to add more info.