Here is a csv list of Splunk .msi filenames, the version reported in Add/Remove Programs, and the Product Code (GUID) for every version (both x86 and x64) of the Splunk windows installer starting with 3.4. If you're curious, historically the upgrade code has always been {9C8D0B7D-C4E6-41C5-94D8-1EA36B03ECE6} but that is subject to change in future versions.
splunk-3.4-44873-release.msi,106.1.13873,{ebd97d5a-0792-4ecc-86c7-3d01c7400104}
splunk-3.4.1-45588-x86-release.msi,106.1.14588,{fb11ef5c-ae02-4589-b9a7-476c540210e7}
splunk-3.4.10-60883-x64-release.msi,106.1.29883,{33236185-1e81-4cf8-a79a-05b5fd1b03f0}
splunk-3.4.10-60883-x86-release.msi,106.1.29883,{5a3cc59b-6b19-4fe3-b9dc-559bc63ca136}
splunk-3.4.11-65313-x64-release.msi,106.1.34313,{2c783903-a15f-49b0-8bd2-1ae785c358d3}
splunk-3.4.11-65313-x86-release.msi,106.1.34313,{1c20b0b7-0b56-4c9c-bd60-175e9ec9db20}
splunk-3.4.12-69236-x64-release.msi,106.1.38236,{adb6d3a7-df9f-4e1b-b524-d916dfae7ad5}
splunk-3.4.12-69236-x86-release.msi,106.1.38236,{3805c1b2-8602-4fd9-a8b1-274a1821b0a1}
splunk-3.4.13-75215-x64-release.msi,106.1.44215,{1e3a8758-69a6-42c2-96b5-da0badfab648}
splunk-3.4.13-75215-x86-release.msi,106.1.44215,{743bc7d7-ad80-4dc4-bce7-71b02066772d}
splunk-3.4.2-46047-x64-release.msi,106.1.15047,{51abb095-9d1a-42e4-90eb-3341dc5f5c01}
splunk-3.4.2-46047-x86-release.msi,106.1.15047,{d26740ba-fe40-45f6-b1d4-430efd9ade2e}
splunk-3.4.3-46779-x64-release.msi,106.1.15779,{054b9ee3-2030-4c8a-8343-0b50660c6ce7}
splunk-3.4.3-46779-x86-release.msi,106.1.15779,{d987b6fa-2499-4102-a43a-915bfd0de060}
splunk-3.4.4-47766-x64-release.msi,106.1.16766,{e6a43ae8-621d-4443-b55b-f6d02acecbd6}
splunk-3.4.4-47766-x86-release.msi,106.1.16766,{b5fe1635-d20e-4131-a55e-d6b7154c03ee}
splunk-3.4.5-47883-x64-release.msi,106.1.16883,{bc4d65f1-9daa-47bd-9d1a-abc7db81e7a1}
splunk-3.4.5-47883-x86-release.msi,106.1.16883,{4728ebbf-b768-4a0c-88d8-b4453f8766ae}
splunk-3.4.6-51112-x64-release.msi,106.1.20112,{e5929c5a-811e-4209-8afc-04bff064fc8d}
splunk-3.4.6-51112-x86-release.msi,106.1.20112,{27b83edb-2f38-4357-9575-e0d19bc55e60}
splunk-3.4.7-53738-x64-release.msi,106.1.22738,{c325146e-9bc6-41d4-adfe-9501ba8f2933}
splunk-3.4.7-53738-x86-release.msi,106.1.22738,{9d88b898-f4ed-42a4-a579-56819f43aa58}
splunk-3.4.8-54309-x64-release.msi,106.1.23309,{92e83d90-77b5-4ac8-91ef-124da4852d08}
splunk-3.4.8-54309-x86-release.msi,106.1.23309,{98955f37-b431-42fd-8989-fdc6c0e4bf60}
splunk-3.4.9-57762-x64-release.msi,106.1.26762,{6be22415-0fd4-4400-9ddb-da57da023541}
splunk-3.4.9-57762-x86-release.msi,106.1.26762,{69c129b2-1772-41e8-811d-7e177378a87b}
splunk-4.0.1-64658-x64-release.msi,107.1.958,{0f2216f7-a37f-429b-8d8a-c4889c093c6a}
splunk-4.0.1-64658-x86-release.msi,107.1.958,{711dfae6-638d-4083-b846-1113025f6d8b}
splunk-4.0.10-77146-x64-release.msi,107.1.13446,{a909f6e1-3c9c-4b17-9650-7ba2677d2d7e}
splunk-4.0.10-77146-x86-release.msi,107.1.13446,{d4e28e91-9a94-49f4-a9f9-eb4585566ff3}
splunk-4.0.11-79031-x64-release.msi,107.1.15331,{96303195-5634-49b4-aa01-480d030e60e3}
splunk-4.0.11-79031-x86-release.msi,107.1.15331,{12d8b9a9-12a5-4366-8fea-d420c0369cf5}
splunk-4.0.2-64889-x64-release.msi,107.1.1189,{69dc358e-ad83-4852-841e-7b342d9a2c4d}
splunk-4.0.2-64889-x86-release.msi,107.1.1189,{c4593ed7-f847-453f-8ed0-77263195e0d2}
splunk-4.0.3-65638-x64-release.msi,107.1.1938,{b624b2df-4ba7-47bd-a03f-8f6776cadf87}
splunk-4.0.3-65638-x86-release.msi,107.1.1938,{0371dbdf-7eb0-4553-9888-8ddc27465028}
splunk-4.0.4-67724-x64-release.msi,107.1.4024,{42742414-f816-4b5f-ba00-1c5868e63a4d}
splunk-4.0.5-69401-x64-release.msi,107.1.5701,{60e2b3f6-78b4-4062-aaef-3312e015e000}
splunk-4.0.5-69401-x86-release.msi,107.1.5701,{755cbfed-ded2-4cb4-8c8e-8a946b09af1a}
splunk-4.0.6-70313-x64-release.msi,107.1.6613,{bffcb22e-81f3-49dc-8263-5892fa5029b6}
splunk-4.0.6-70313-x86-release.msi,107.1.6613,{d7ec40d0-8bef-4a44-a09e-d877d40347bb}
splunk-4.0.7-72459-x64-release.msi,107.1.8759,{31fcfb71-872f-422e-a3c0-b41fcade5cb0}
splunk-4.0.7-72459-x86-release.msi,107.1.8759,{1cf5a02a-ddb7-4192-be0c-a51eb7e98d0e}
splunk-4.0.8-73243-x64-release.msi,107.1.9543,{4cd47afe-bd71-45d2-b1ed-0cf2fd58dc8b}
splunk-4.0.8-73243-x86-release.msi,107.1.9543,{2c0fae08-7c9c-40f9-ba21-82a2aad07f0d}
splunk-4.0.9-74233-x64-release.msi,107.1.10533,{31915343-9dc9-47ea-b146-31ab106855ef}
splunk-4.0.9-74233-x86-release.msi,107.1.10533,{c0d987d9-093f-4c7a-935b-e8a2da6d4a7f}
splunk-4.1-77833-x64-release.msi,107.1.14133,{a3d0f20f-03a8-4670-a659-d611b1bf8c5f}
splunk-4.1-77833-x86-release.msi,107.1.14133,{443f4435-18ff-4a2e-a87f-8c7f59c6c520}
splunk-4.1.1-78281-x64-release.msi,107.2.14581,{deea63a9-9dc7-4917-8f2e-f504898a4626}
splunk-4.1.1-78281-x86-release.msi,107.2.14581,{8b9411f9-4b01-49db-90b8-5f9f03d7b427}
splunk-4.1.2-79191-x64-release.msi,107.2.15491,{d6227683-9184-4ef0-9e34-79251b7c9c9d}
splunk-4.1.2-79191-x86-release.msi,107.2.15491,{78d6f15d-2645-4c7a-a677-36e6b4ddbd5c}
splunk-4.1.3-80534-x64-release.msi,107.2.16834,{67debc74-05c7-4bd0-8c1b-127e8316e6f3}
splunk-4.1.3-80534-x86-release.msi,107.2.16834,{d946931c-9e7a-424d-a4bf-c564df404caa}
splunk-4.1.4-82143-x64-release.msi,107.2.18443,{a614d848-4be8-4415-96f9-94613bbf60ee}
splunk-4.1.4-82143-x86-release.msi,107.2.18443,{a4f0f1ae-29a5-48fe-a370-c7eedb3e09a4}
splunk-4.1.5-85165-x64-release.msi,107.2.21465,{bd9247a9-2f6f-493b-ae49-157950a968dc}
splunk-4.1.5-85165-x86-release.msi,107.2.21465,{f3319ff9-76ea-4917-9541-614b8e980454}
splunk-4.1.6-89596-x64-release.msi,107.2.25896,{b15e21f0-ae46-497d-8107-b92b6a5a1c6b}
splunk-4.1.6-89596-x86-release.msi,107.2.25896,{298ef7be-e0f5-4270-b68a-e811d2f1f6a3}
splunk-4.1.7-95063-x64-release.msi,107.2.31363,{814459fa-a522-4f0f-bcf1-a0d386a86703}
splunk-4.1.7-95063-x86-release.msi,107.2.31363,{655c80f5-386d-4378-bf60-78680da9b1fb}
splunk-4.1.8-97242-x64-release.msi,107.2.33542,{a2bf9e4d-d3bc-41f6-a1ac-306254e75e61}
splunk-4.1.8-97242-x86-release.msi,107.2.33542,{135e3ae7-6f53-4718-99e3-164b08d8a1ea}
splunk-4.2-96430-x86-release.msi,108.1.32730,{9df62ab9-d684-41db-b38f-cc19bfa2f3a0}
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