If it’s not a regular thing just do it on convoi ex permit. 4.5 is a doddle in France 
Like the UK, France has no height limit, BUT Euro rules are 4.0m.
I don’t know the usual height of autoroute bridges etc. I remember that some bridges on the A25 (?(Dunkerque - Lille)) are 4.0 or slightly lower.
You’ll need to get the large scale Michelin map of France and check your planned route very carefully. The large scale map has bridge heights marked on it.
I worked for the Scottish office of a French company for a while. We had adjustable height trailers and frequently ran at 4.5 - 4.6m in France. Some of the detours were well weird. Off the major national onto really small department roads to get to the tall bridge then back to the national.
Simon, your avatar has triggered a longing for a glass of Scotch - but I’ve never stopped at one. End up getting wrecked on the stuff! Its a beautiful drink but it hates me.
as billy said anything up to 4.5m is not a problem in France if it is not marked it will be over 4.5m
mazzer:
as billy said anything up to 4.5m is not a problem in France if it is not marked it will be over 4.5m
They well may possibly be the case, but the rule is If it is not marked it will have a minimum height of 4.3m.
Edit. After further investigation I have found the following additional info.
La hauteur libre minimale sous ouvrage d’art est:
— 4,30 m sur l’ensemble du réseau routier national, départemental et communal;
— 4,50 m sur les grandes routes de trafic international décrites dans l’accord de
Genève du 15 novembre 1975 (AGR) et précisées par décision ministérielle;
— 4,75 m sur les autoroutes
So whilst 4.3m is the normally quoted minimum, on the autoroutes it should be 4.75m.
Héraultais:
So whilst 4.3m is the normally quoted minimum, on the autoroutes it should be 4.75m.
Ahhh, that’ll explain those low bridge height markings on the A25 
If it was me I’d go with the 4m trailer, then you know you’re ok. Also many circuits in Europe have tunnels to get under the track into the paddock and I doubt they’d be anywhere near 4.6m. So you could avoid low bridges all that way only to find that you can’t actually get your truck behind the garage. 
Or we could do it for you, we have a full O’licence and high value insurance for both cars and equipment in transit and many years experience of transporting racecars to circuits across Europe.