Missing InstanceContext when upgrading from Glass.Mapper.Sitecore to Glass.Mapper.Sc

I am upgrading a project from Glass Mapper v2 (Glass.Mapper.Sitecore) to v4 (Glass.Mapper.Sc) and I ran into an issue where our solution used an InstanceContext object to get classes by template id. I couldn't find this InstanceObject or similar class in the new Glass Mapper. Here is a piece of code using it, hopefully it shows where I went wrong.

protected InstanceContext InstanceContext;

public MappingService(ISitecoreContext context)
{
    InstanceContext = context.InstanceContext;
}

public T Map<T>(ISitecoreItem sourceItem) where T : class
{
    Type sourceType = InstanceContext.GetClassById(sourceItem.TemplateId);
    if (sourceType != null)
    {
        return Map(sourceItem, sourceType, typeof(T)) as T; 
    }

    return Mapper.Map<T>(sourceItem);
}

// The GetClassById is implemented in an extensions class like this:
public static Type GetClassById(this InstanceContext context, Guid templateId)
{
    if (context.ClassesById.ContainsKey(templateId))
    {
        SitecoreClassConfig config = context.ClassesById[templateId].FirstOrDefault();
        if (config != null)
        {
            return config.Type;
        }
    }
    return default(Type);
}