Element composition (to two dp): carbon = 92.24%     hydrogen = 7.76%

Empirical formula = CH and molecular formula = C₄H₄

(1) cyclobuta-1,2-diene, , skeletal formula

A cyclic diene i.e. two alkene groups in the very strained ring.

Also called 1,2-cyclobutadiene and very unstable if it exists, can it exist?

Functional group (ring not open chain) and positional (1, 2 for C=C x 2) isomer.

(It is possible to stabilise cyclobutyne, which will have an equally very strained ring!, see (6))

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 3 ¹³C (email if disagree?)

¹H NMR ratio of peaks: 2 (1+1) : 2 = 1 : 1 (for equivalent protons)

Index of ¹H NMR spectra organic compounds and Index of ¹³C NMR spectra organic compounds

 

(2) cyclobuta-1,3-diene, , skeletal formula

Sometimes just called cyclobutadiene, a cyclic diene i.e. two alkene groups in the ring.

Also called 1,3-cyclobutadiene and very reactive and highly unstable, but does exist, albeit with a very short life-time!.

Functional group isomerism (ring not open chain) and positional (1, 3 for C=C x 2) isomer.

Number of low resolution NMR chemical shift δ signal peaks: 1 ¹H and 1 ¹³C (email if disagree?)

 

(3) 3-methylenecyclopropene, , skeletal formula

Also simply called methylenecyclopropene.

It is both a cyclic and non-cyclic alkene (C=C group).

Functional group (ring not open chain) and positions of C=C groups isomers.

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 3 ¹³C (email if disagree?)

¹H NMR ratio of peaks: 2 (1+1) : 2 = 1 : 1 (for equivalent protons)

 

(4) but-3-en-1-yne, H₂C=CH-C≡CH, , abbreviated structural formula and skeletal formula

Other names: I think the above is the preferred IUPAC name. Once called vinyl acetylene.

1-buten-3-yne, but-1-yne-3-ene, 3-butene-1-yne, but-1-en-3-yne, but-1-en-3-yne

Old name: vinylacetylene

(4) has two functional groups, a double bond C=C alkene group and a triple bond C≡C alkyne group.

Functional group isomer, open chain, not ring, and alkyne group.

Number of low resolution NMR chemical shift δ signal peaks: 3 ¹H and 4 ¹³C (email if disagree?)

¹H NMR ratio of peaks: 2 : 1 : 1 (for equivalent protons)

BUT, for the 'end' =CH₂ alkene protons, you can get two chemical shifts close together, if there are two different groups attached to the other carbon of the C=C bond i.e. R"R'C=CH₂ where R" and R' are different. This causes the two =CH₂ protons to experience slightly different fields.

 

(5) buta-1,2,3-triene, H₂C=C=C=CH₂, , abbreviated structural formula and skeletal formula

Also simply called butatriene, with a triple consecutive arrangement of three alkene groups.

Number of low resolution NMR chemical shift δ signal peaks: 1 ¹H and 2 ¹³C (email if disagree?)

 

(6) cyclobutyne, , theoretical skeletal formula

An alkyne group as part of a alicyclic compound with an extremely strained ring.

Stable molecule not isolated but can be stabilised as a ligand in metal complexes (university level stuff).

Number of low resolution NMR chemical shift δ signal peaks: 1 ¹H and 2 ¹³C (email if disagree?)

 

(7) 1-methylcyclopropa-1,2-diene

Theoretical, and highly unlikely to exist with an extremely strained bond system of the three membered cyclic carbon ring [-C=C=C-].

 

Examples (1-3 and 6-7) are all alicyclic unsaturated compounds.

Examples (4-5) are open chain highly unsaturated non-cyclic aliphatic compounds.

Examples (1-7) As far as I can tell, non of these selected constitutional isomers can exhibit stereoisomerism i.e. cannot be E/Z (geometrical) or R/S (optical) isomers.

 

(8-) You find many more on https://enAs far as .wikipedia.org/wiki/C4H4

and https://www.researchgate.net/figure/Scheme-1-Grouping-of-C-4-H-4-structures-to-be-expected-on-the-C-4-H-4-PES-The-horizontal_fig1_26877324

quite a few more isomers of C₄H₄ and quite a variety of structures you are unlikely to come across pre-university.