By considering Coulomb systems with continuously changing charges of t
he particles involved, it is shown, with the aid of the Hellmann-Feynm
an theorem and the virial theorem, that the energy of a neutral four-p
article Coulomb system is approximately equal to half the sum of the e
nergies of all its three-particle subsystems. This rule is verified fo
r all 35 symmetric four-particle mesic molecules X+/-X+/-Y-/+Y-/+, who
se energies are presently known, and its accuracy is found as a functi
on of the particle masses. With allowance for correction factors depen
ding on the particle masses, the nonrelativistic energies of asymmetri
c four-particle Coulomb mesic molecules d(+)t(+)p(-)p(-), p(+)K(+)pi(-
)pi(-),d(+)K(+)pi(-)pi(-), d(+)K(+)mu(-)mu(-), t(+)K(+)mu(-)mu(-), pi(
+)mu(+)mu(-)mu(-),pi(+)mu(+)pi(-)pi(-), and pi(+)mu(+)pi(-)mu(-) are p
redicted on the basis of this rule. It is shown that the energy of a n
eutral N-particle Coulomb system is approximately equal to the ratio o
f the sum of the energies of all its (N - 1)-particle subsystems (ther
e are N such subsystems) to (N - 2). This rule is verified for the bos
on analog of the six-particle positronium molecule e(+)e(-)e(+)e(-)e()e(-).