What am I doing wrong with this FFT?

[pyhill00]

Hi everyone,
I am trying to just FFT four cosines but the results are weird.

This code below gives the following figures:

https://imgur.com/a/FP3ylak

import numpy as np

import scipy.fftpack
from scipy.fftpack import fftfreq
from scipy.fft import fft
from scipy.fft import fft2
from scipy.fft import fftn


from scipy.signal import blackman, flattop, boxcar
import matplotlib.pyplot as plt
import random

plt.rcParams['figure.figsize'] = [9, 5]

gamma = 2.8e6 # short for gamma/2*pi, in Hz, [Hz/G]
tau = 1e-6 # us;
G = np.linspace(0, 1.7e6, 2400) # G_max is 1.7G/um ~ 1.7e6G/m
k = gamma*G*tau # 1/nm


rx = np.linspace(404e-7,416e-7,4)
outer = np.outer(rx,k)

y = 0.5*np.cos(2*np.pi*outer) #+ np.random.normal(0, .1, k.shape)
yy = np.sum(y,axis=0)

plt.figure(0)
#
#plt.xlim(1.0e8, 1.3e8)
plt.plot(k, yy)


f = fftfreq(len(k), np.diff(k)[0])
yf = fft(yy)
plt.figure(1)
plt.xlim(0, 500e-7)
plt.plot(f[:G.size//2], np.abs(yf[:G.size//2]))

r_max = f[:G.size//2][np.argmax(np.abs(yf[:G.size//2]))]
amplitude_max = np.abs(yf[:G.size//2]).max()

I would have figured, that the FFT would be just 4 spikes but not something like this. If I change rx to rx = np.linspace(404e-7,4016e-7,4) I get this:

https://imgur.com/a/ZjTedft


So somehow the spacing is ruining the FFT.