If you just chop 4cm of length off the front of the top tube (and downtube), you then also need a longer headtube and longer fork steer tube. If you take the length off the back end of the top-tube (and dont change the angles of the downtube), you also need a shorter seat-tube and new/repositioned seatstays due to the lowered height of the seatube lug. Either way, this will have the effect of raising the entire front end of the bike upward, changing (relaxing) the frame angles and raising the bottom bracket height.
To keep the frame angles constant, you would need to have a shorter headtube and a shorter seat-tube. THis would require removal,replacement or modification of the toptube, downtube, seatstays, headtube and fork steer tube. This is major surgery and would not be cost effective. Try riding your existing bike with a much shorter stem first to see if you even like the suggested reach. If so, find a smaller replacement frame with shorter top-tube to get you the stem+top tube reach that you want. Only potential downside of a shorter seattube measurement is that your range of adjustment for the front stem/handlebar height also drops lower.