Greg, the limiting factor for the most part is the valve train. Stock valve springs apply pretty low pressure on the valves for durability of the entire valve train. At high rpm harmonics becomes a factor and the valve springs actually bounce because they don't have enough pressure to hold the valve on its seat at that point. The valving in the hydraulic lifters also can't support this punishment. End results are valve float and lifter collapse. On an engine with a lot of miles it gets worse as the valve springs have lost some strength and the lifters might be slowly leaking by.
To get a better idea of how far any valve train can run before the valves float, check a cam mfrs
spec book, like Crane, Comp, Isky, etc. Dig far enough into it and you get the cam rpm range and the spring float info as well.
I'd consider 4500rpm not a problem for a bone stock 250 and 4800 the absolute max once in a while if the engine is in good condition.
My 250 has a Comp Cams 260H with their recommended springs and retainers along with Crane Gold roller rockers. The engine is fresh and I occaisionally buzz it up to 5300 with no problems. This engine is also not done to stock specs.
