SetDirectory["C:\\drorbn\\AcademicPensieve\\People\\Alexakis"];
det=Get["det.m"]

d2=Together[det]

Cases[d2, _Symbol, ∞] // Union

d2 /. {a41->1,a42->1,a43->1,a44->1}

d3=(5.4027234947288059323838154849`12.21532680880618*^-665 a41 (a42^3) a43+5.4027234947288823415832552348`12.612241203515852*^-665 (a42^4) a43+2.99127720274966042763735784828`12.416962792450057*^-664 a41 (a42^4) a43+2.00066112773002111728168419739`12.757612726351578*^-664 (a42^5) a43+6.13783411218722277029650494044`12.488189308897063*^-664 a41 (a42^5) a43+2.46952298034811189432977945771`12.75391391907731*^-664 (a42^6) a43+5.54408060527016095468163848136`12.485603396104278*^-664 a41 (a42^6) a43+1.01608807640950657334612635362`12.600442328076644*^-664 (a42^7) a43+1.86304774454785431349945767891`12.37174817959532*^-664 a41 (a42^7) a43-5.6127596054697535404954084095402811`12.916717685822091*^-679 a41 a42 (a43^2)-6.0943005288896929160833329856131461`13.094635890097939*^-679 (a42^2) (a43^2)+4.9758275042370104259637622618`12.401119168977544*^-665 a41 (a42^2) (a43^2)+5.402723494728789708049024886`12.744238527116044*^-665 (a42^3) (a43^2)+3.63801227608928046518399425828`12.362607308186686*^-664 a41 (a42^3) (a43^2)+2.94489845073899304932043725106`12.683526517261956*^-664 (a42^4) (a43^2)+1.054046437792869885286785804475`12.31838026781235*^-663 a41 (a42^4) (a43^2)+5.96609079348713956670812312416`12.604470880813269*^-664 (a42^5) (a43^2)+1.513212089655319640164319296866`12.261496290215117*^-663 a41 (a42^5) (a43^2)+5.33208864602600862430846768845`12.485247960991813*^-664 (a42^6) (a43^2)+1.076994672732628523809321834616`12.174712253058571*^-663 a41 (a42^6) (a43^2)+1.77582340859438087304286150836`12.25477252992247*^-664 (a42^7) (a43^2)+3.04197455690494502657727570313`11.9959109616945*^-664 a41 (a42^7) (a43^2)+4.9758275042371140691441972215`12.190902536612464*^-665 a41 (a42^3) a44+4.9758275042371753042983740389`12.591437983939832*^-665 (a42^4) a44+2.75492154887454689743989784644`12.393702277341179*^-664 a41 (a42^4) a44+1.84257896516999992148976950666`12.737591547995514*^-664 (a42^5) a44+5.6528533843464649858124995931`12.465346825301848*^-664 a41 (a42^5) a44+2.27439371641922382345481363471`12.733650705948588*^-664 (a42^6) a44+5.10601528809044939156710178074`12.462340275819164*^-664 a41 (a42^6) a44+9.3580191587790650421764924495`12.578896923256307*^-665 (a42^7) a44+1.71583909820160904911028036978`12.346716772841841*^-664 a41 (a42^7) a44-5.6127596054696046091854411668121808`12.955964195524842*^-679 a41 a42 a43 a44-6.0943005288895952626512657836175372`13.150680004032335*^-679 (a42^2) a43 a44+4.9758275042368129725340393027`12.323049249421114*^-665 a41 (a42^2) a43 a44+5.4027234947286394405100817504`12.666865798986462*^-665 (a42^3) a43 a44+4.48187903336367401167483470229`12.365376322746036*^-664 a41 (a42^3) a43 a44+3.79609443864930024133191319001`12.703248426095394*^-664 (a42^4) a43 a44+1.521011428886112891708136026576`12.381757089588072*^-663 a41 (a42^4) a43 a44+9.1181209256110554905894660178`12.689115704544449*^-664 (a42^5) a43 a44+2.470967999813723510142671050048`12.372263084484352*^-663 a41 (a42^5) a43 a44+9.22280793783409866727442332777`12.617826822500563*^-664 (a42^6) a43 a44+1.941792615590002458545900307146`12.324148518568142*^-663 a41 (a42^6) a43 a44+3.37666434529438831005505320592`12.42561045372966*^-664 (a42^7) a43 a44+5.94719927664630882159604251731`12.178831773036283*^-664 a41 (a42^7) a43 a44+4.451524187413007767714344881488651276609`13.477612443355834*^-693 a41 (a43^2) a44+4.833438115411048907771738660525234046129`13.477612443355834*^-693 a42 (a43^2) a44-1.42376406308444256548505137025807937`12.836779523716007*^-678 a41 a42 (a43^2) a44-1.54591443293136651818036267568556908`13.014189083493873*^-678 (a42^2) (a43^2) a44+9.1234258370445815891385836226`12.28319182368749*^-665 a41 (a42^2) (a43^2) a44+9.9061607501958274385114633621`12.632873783218713*^-665 (a42^3) (a43^2) a44+6.54069274651126894160084698434`12.248315892630309*^-664 a41 (a42^3) (a43^2) a44+5.28578384808557288995093628266`12.575183099391314*^-664 (a42^4) (a43^2) a44+1.86106491813373684310794596542`12.206855673410217*^-663 a41 (a42^4) (a43^2) a44+1.051759164811132317454408078826`12.498299241261579*^-663 (a42^5) (a43^2) a44+2.628123484755649927540495254909`12.151987651123699*^-663 a41 (a42^5) (a43^2) a44+9.25633012657975520160606070921`12.38080218961779*^-664 (a42^6) (a43^2) a44+1.842798262537768269813540974578`12.06656861345375*^-663 a41 (a42^6) (a43^2) a44+3.04197455690496372901940237193`12.15191578565244*^-664 (a42^7) (a43^2) a44+5.13523269303961018981635775619`11.888496628628777*^-664 a41 (a42^7) (a43^2) a44)/((((1.1088504383096044`*^-204-9.83018852909682`*^-191 a42))^2) (((5.100866449633201`*^-97+6.296272138474589`*^-97 a42))^3))

d3 /. {a41->1,a42->1,a43->1,a44->1}