It is not possible to convert MXN (OpenColor multicolour profile) or MXD (OpenColor multicolour profile with DotProof) to MX4, as these profiles use Spectral data, whilst MX4 profiles require Lab target data.
The only real option available is to take the original test chart from the print run and build an MX4 profile separately, from scratch. This would require an ECI2002 or FlexoV2 chart, as ColorProof ProfileEditor doesn't support the smaller OpenColor strips.
If you were to build the MX4, the chart would need to be printed in the format for a measurement device that you have.
MX4 is only able to support 4 colours (CMYK), whilst MXN supports spot colors (for a multi-colour profile) too. Any spot color results with the MX4/db3 may not look as close as the result from an MXN profile.