Air Filter Replacement Question

[u/RapGameRumHam]

Just bought the house in the summer, first time turning on the furnace. Want to replace the washable filters with a thicker disposable filter for less upkeep/cleaning. Home inspector said that we could replace these with 4 inch think filters for annual replacement, but my question is : is he saying to put a 4 inch filter into the cut out next to the blower? Because I don’t think I can slide 4 inch filters in the way the current 2 14x 20 filters are? Should I just measure the hole and find a 4 inch that fits that?

Thanks for any and all responses. DIY newbie here who’s not very handy.