Amp / preamp for macintosh xt50

XR 50 

In that price range, a McIntosh Integrated would be perfect.  You won’t get much in separates for $3-4K, especially if you want McIntosh. 

@stereo5   thank you ..
I do not want mcintosh gears .. what else brands might work good with the XR 50 ? 

Those speakers are a 81db low efficient speaker. You will need a power amp capable to drive them. The Van Alstine Vision Set 400 (225 watts) will do the trick. Match that with a Van Alstine preamp and you will be in the ballpark of your $4000 budget!
https://avahifi.com/

@yogiboy  thanks man .. 
How about replacing the van alstine preamp with doge 8 clarity preamp ?

You can do that. A tube preamp would be a nice fit. Another nice option for a power amp within your budget is Odyssey Audio!
https://odysseyaudio.com/