WARNING - This post contains references to decibels and other serious radio content, viewer discretion is advised.
The accepted formula for calculating path loss is:
Pathloss = 32.45 +20Log(f) + 20Log(d)
Where
Pathloss is in dB
f is in Mhz
d is in km
This formula is applicable to "long" paths, hence d in km.
I have no idea how well is scales to metre distances, but lets try it anyway.
The pathloss at 433Mhz over 300metres is 74.7dB
The pathloss at 433Mhz over 5metres is 39.2dB
The difference is 35.5dB.
So I suggest that you try a 30dB attenuator to start with.
On a practical note, if you are expecting this thing to work reliably and repeatedly at 4.9m, but never work at 5.1m, you are in for a surprise.
The range will be very much affected by surrounding objects, how well the transmitter and receiver are screened, how well the supply leads are filtered, etc, etc.
JimB