4000 RPM and no load is totally different compared to 4000 RPM under a load,IMHO.
The vacuum rating is different under those situations and the outer carbs rely on vacuum to operate when the engine is under a load..

'68 Newport Custom Barge on a Budget!! BOAB
1973 Satellite WAGON! 318- 3 on the tree!!
2008 Chrysler 300c HEMI!