Just pay attention to the fact that whenever you climb any hill, it can be very windy over there. This something you wouldn't notice within the royal compound of Ambohimanga (probably thanks to the egg & mud enclosure walls). Yet, I happened many times to be too lightly covered for the wind when sitting on this Ambatomiatendro rock. The top of the primary forest is really near the rock itself.
The risk ? Nothing serious except a headache & catching cold being exposed to both some bright sun & strong wind.
You don't need a 4wd to go there.
You may hire a bus but I can't advise where to catch it. But I know there are regular lines (not as frequent as in European standards though...).
So either you hire a taxi, a guide or simply a car without any driver... or a bicycle.. or a motorbike (a very cool option!!!).
The road is good though somewhat crowded near the urban center but it becomes so desert as you get North... especially after Sabotsy-Namehana, it's so calm and sparse.
Ambohimanga, well, it's another city, that lives on its reputation of a sacred place... no business center, no supermarkets.. it's a countryside town... We use to say that Ambohimanga is a royal city.. but it's more of a town, really... very serene; just crowded at Ambatomitsangana gate where are the Ambohimanga daily (weekly ? - anyway, some stalls, not a big market) market, the bus station, the administrative bureau, the church.etc..
Anyway, the 20 km road is generally good, more and more sparse as you head North and you would be tempted to speed up, happy to escape the chaotic and messy Tana, its downtown hassle and the traffic jams... and you speed up, like many others..
That's the warning: sometimes, you would see phantom cars as you tackle a turn, you would be surprised by a oxcart you wouldn't expect, another crazy driver driving in the middle of the road, not keeping his right... So watch out that speed adn stay focused on the driving though the landscape is so inviting !
How do I know that ? I used to commute 4 times a day on this route for nearly 14 years (Malaza, my home village is 5 km from Ambohimanga - 20 km from urban Tana).