NB: ArkhamDB now incorporates errata from the Arkham Horror FAQ in its card text, so the ArkhamDB text and the card image above differ, as the ArkhamDB text has been edited to contain this erratum (updated August 2022): Erratum: The purchase restriction on this card should be replaced with the keyword: "Researched." - FAQ, v.2.0, August 2022

The rules reference on Experience states "When purchasing a higher level version of a card with the same title, the investigator may choose to ‘upgrade' that card by paying only the difference in experience (to a minimum of 1) between the two cards and removing the lower level version of the card from his or her deck." No part of this passage allows you to "side-grade" from one level 4 Strange Solution to another level 4 solution for 0 experience. In fact, you would have to pay 1 xp to swap the level 4 strange solution out for a level 0 strange solution, then upgrade that for 4 xp.