It is puzzling the use of 4140 annealed, because its properties can be easily met by unalloyed steel.
If 4140 was selected, though, it may be meant to be heat treated, while obviously the 1018 welded part would not improve its properties.
Preheating can be used to reduce the self quenching (cooling) rate and reduce the hardness of the 4140 heat affected zone, possibly avoiding cracking.
Postheating would temper the hardened structures to lower and more acceptable values.
If heat treatment is prescribed for the welded 4140 then pre and post-heating are redundant, provided welding is done without cracking.
