Quantitative measurement of ring strain
Let's look at following table from a Yale website:
$$
\begin{array}{c|ccc}
 &amp; \ce{(CH2)3} &amp; \ce{(CH2)4} &amp; \ce{(CH2)5} &amp; \ce{(CH2)6} &amp; \ce{(CH2)7} \\
\hline
\Delta H_\mathrm{Combusion} \ (\pu{kcal/mol}) &amp; -499.8 &amp; -656 &amp; -793.5 &amp; -944.6 &amp; -1108.3  \\
\Delta H_\mathrm{Combusion} \text{ per } \ce{-CH2}-  \ (\pu{kcal/mol}) &amp; -166.6 &amp; -164 &amp; -158.7 &amp; -157.4 &amp; -158.3 \\
\text{Ring strain per } \ce{-CH2}-  \ (\pu{kcal/mol}) &amp; 9.2 &amp; 6.6 &amp; 1.3 &amp; 0 &amp; 0.9 \\
\text{Total ring strain}  \ (\pu{kcal/mol}) &amp; 27.6 &amp; 26.3 &amp; 6.5 &amp; 0 &amp; 6.3 \\
\hline
\end{array}
$$
Now, just concentrate on ring strain values for cyclopropane and cyclobutane only:

Ring strain per $\ce{-CH2}-$ (X-values) are $9.2$ and $\pu{6.6 kcal/mol}$, respectively.
Total ring strains (Y-values) are $27.6$ and $\pu{26.3 kcal/mol}$, respectively.

According to Y-values, cyclopropane and cyclobutane are equally unstable (relatively speaking), while according to Y-values, cyclopropane is significantly less stable than cyclobutane. Which valuse should be more reliable? Let's look at experimental findings:

Although cyclopropanes are far less reactive than alkenes, they react with chlorine and bromine under polar conditions to form corresponding addition products, 1,3-dihalopropanes, as the predominant product (Ref.1). To my understanding, cyclobutane and higher cycloalkanes do not give this ring opening reaction.
Cyclopropane also reacts with HCl and HBr to form corresponding addition products, 1-halopropanes (Ref.2; Reactions of bromocyclopropane with hydrobromic acid has given mixture of 1,1-, 1,2- and 1,3-dibromopropanes). Again, to my understanding, cyclobutane and higher cycloalkanes do not give this ring opening reaction.

So which one you think should be more reliable and why?
References:

Joseph B. Lambert, Erik C. Chelius, William J. Schulz, Jr., Nancy E. Carpenter, “Polar bromination and chlorination of cyclopropane,”  J. Am. Chem. Soc. 1990, 112(8), 3156–3162. (https://doi.org/10.1021/ja00164a043).
Choi Chuck Lee, Bo-Sup Hahn, Kwok-Ming Wan, D. J. Woodcock, “Reactions of cyclopropane with hydrochloric acid and of bromocyclopropane with hydrobromic acid,” J. Org. Chem. 1969, 34(10), 3210–3211. (https://doi.org/10.1021/jo01262a097).