How to filter specific rows from large data file

[Ariane]

Hi I have a large data file and I'm only interested in rows with specific x values of 4.125 as shown below. Now because the value of 4.125 relates to the stop position of the ion, the corresponding start position is also of interest to me and I want to keep this information in the array. How do I write a program which effectively finds the x stop position of 4.125 and retains the ion start position. It is a 120982 * 9 array and in the example shown below I would be interested in keeping the information of ion # 3849096.

"Ion N","Mass","Charge","X","Y","Z","Azm","Elv","KE" 3849094,0.00054858,-1,66.5216,-51,-3.8,-180,88.7,18160 3849094,0.00054858,-1,27.3925,30.3532,-4.07076,-177.1,41.5494,17697.2 3849095,0.00054858,-1,66.5216,-51,-3.7,-180,88.7,18160 3849095,0.00054858,-1,26.6277,31.0039,-3.91402,-177.096,40.8293,17699.4 3849096,0.00054858,-1,66.5216,-51,-3.6,-180,88.7,18160 3849096,0.00054858,-1,4.125,44.9887,-2.47517,-176.363,25.715,17711.1

This is the code I have developed so far but does not work:

import pandas as pd 
import numpy as np

opts = pd.read_csv('Ambre_2.dat',sep = ',', low_memory = False)
df = pd.DataFrame(opts)

X = df.iloc[:,3]
IonN = df.iloc[:,0]
tol = 1e-6
Fltr = 4.125

filterreddata = df[abs(df.X-Fltr)<tol,:]
filteredions = df(np.in1d(df.IonN, filterreddata.IonN), :]
filteredions[2:2:end, :] = []
f = open('ions.csv', 'w')
f.write(tabulate(filteredions))
f.close()