Originally Posted by Tony P

The track north if Zhilagalovo is fine graded gravel. A very enjoyable road.

RoB.
Any car, 2 or 4WD should have little difficulty on the Federal Road parts of the RoB network of roads and tracks. A 4WD would be needed once you get off that and some sections would probably be impassable even to them.
Much depends on the weather and amount of water lying about, not just rain but the quantity of the past winter's snow. When it eventually melts the permafrost below the surface stops water absorbtion so it just sits there slowly evaporating. Hence the whole area and all of northern Siberia, being so wet and muddy in summer and smooth zimniks - frozen ice tracks only passable in winter.

BAM (Western)
Different proposition!
Consider this - about 100 or 200km west of Taksimo we met Michael Rej coming the other way. He is probably Poland's most experienced 4x4 expedition leader having been organising and leading them and instructing for more than 15 years. A typical 'hard core' Polish adventurer - and at the top of his trade.
This time he was leading SyberiaMongolia 2009 - Syberia-Mongolia 2009 a two vehicle (Discovery and a Toyota something) trip to Mongolia. They were very well sponsored by many Polish and World brands and a Polish TV - so money equipping themselves with the most practical equipment was presumably not a problem.
They chose to do the western BAM as a diversion to relieve the monotony of the Trans Siberian Highway to their intended Mongolia turn off.

They eventually got through to Tynda but by then the vehicles were in such a bad state that their Mongolia aspirations (and the purpose of the trip) were curtailed and they had to work very hard just to get their vehicles back to Poland, not on trailers.

There are many of their clips if you do a YouTube search syberiamongolia as well as through their own WebSite above.

There are very steep railway embankments you must climb (and descend) to get over some rivers on the railway bridges (active with freight trains) - you will have to drive along the actual rails at times - many 'road' bridges were not even safe for a our single track light bikes - others we had to rebuild with timber to make just a single track for the bikes - others were partially or completely absent, requiring some very bumpy river crossings, dry and wet. And so on. And it will all have deteriorated further by 3 more Siberian winters by next summer.

And then there is the VITIM BRIDGE - something I personally never ever, EVER want to see again in this life (I am still sometimes kept awake at night thinking about it). Without that bridge I would dearly love to revisit the entire BAM but with a lighter bike and less stuff.

BAM (eastern section)
4 wheels? Forget it is my opinion - although I admit to knowing nothing of the full capabilities of 4x4s.

I am not trying to paint a picture to glorify out own exploits but giving honest opinions. Look at Michael Rej's BAM drive - and be aware of the atrition to the vehicles that he suffered, such that they barely got them home.