There are several hundred 6 letter words with I as the 4th letter, including every 6 letter word ending in -ing (e.g. aching, acting, aiding, ailing, asking, awning, baring, caring, daring, during, fixing, flying, giving, having) and -ion (action, bunion, cation, fusion, lotion, motion, nation, notion, option, potion, ration, vision)
Some of the other common words are these for the letter A:
achier
active
addict
admire
admits
adrift
advice
aerial
affirm
airily
albino
allied
allies
alpine
antic
aphids
arrive
ashier
aspire
assign
atrium
attire
audits
avails
avoids
aweigh
aboardattainattackcanardcreamydamagedebatedepartdetachdisarmdreamyembalmembarkentailescapeexhaleexpandgaragegreaseharassheraldhumanehomageimpactimpartinfantinhaleinnateinsaneintakemanagemirageparadephrasepleaserecallregardrelaterepairremainremarkrenameresaleretakeretainscrapesheathsilagesprawlshrankstraitstrainthrashtruantunfairunmaskunpackunpaidwreath
lawyerfrayedgrayermartyrplayerprayedprayerpraysslayedredyeredyedslayerstayedswayedtrays
walkedwalkerweakenweakerweaklyweeklywickedwinkedworkedworker
scamps
Some six letter words with the fourth letter U are:absurdaccuseacquitactualadjustambushamountannualaroundarouseassumeassureastuteaugustautumnbeautyblousecasualclausecolumncrouchdeductdefusedeludedelugedeluxedemuredenudedeputydiluteendureengulfenoughensureexcuseexhumefigurefuturegerundgrouchgroundgrouseimmuneimmureimpureimputeinduceinductinfuseinjureinsultinsurejaguarjejunejujubekabukilegumeliquidliquorlocustluxurymanualmanurematureMedusaminuetminutemisusemodulemutualnaturenebulanoduleobtuseoccultoccupyperuseploughpseudorebuffrebukereducerefuelrefugerefundrefuserefuteresultresumereturnrobustrotundsalutesecureseducesequelsequinsexualshouldstrucksuburb
aboardattainattackcanardcreamydamagedebatedepartdetachdisarmdreamyembalmembarkentailescapeexhaleexpandgaragegreaseharassheraldhumanehomageimpactimpartinfantinhaleinnateinsaneintakemanagemirageparadephrasepleaserecallregardrelaterepairremainremarkrenameresaleretakeretainscrapesheathsilagesprawlshrankstraitstrainthrashtruantunfairunmaskunpackunpaidwreath
Shoots.
lawyerfrayedgrayermartyrplayerprayedprayerpraysslayedredyeredyedslayerstayedswayedtrays
walkedwalkerweakenweakerweaklyweeklywickedwinkedworkedworker
scamps
blurryblurtschurchchurnsflurryfourthhourlymournspouredscurryslurpssouredspurnssturdytoured
Some six letter words with the fourth letter U are:absurdaccuseacquitactualadjustambushamountannualaroundarouseassumeassureastuteaugustautumnbeautyblousecasualclausecolumncrouchdeductdefusedeludedelugedeluxedemuredenudedeputydiluteendureengulfenoughensureexcuseexhumefigurefuturegerundgrouchgroundgrouseimmuneimmureimpureimputeinduceinductinfuseinjureinsultinsurejaguarjejunejujubekabukilegumeliquidliquorlocustluxurymanualmanurematureMedusaminuetminutemisusemodulemutualnaturenebulanoduleobtuseoccultoccupyperuseploughpseudorebuffrebukereducerefuelrefugerefundrefuserefuteresultresumereturnrobustrotundsalutesecureseducesequelsequinsexualshouldstrucksuburb
sachet
ladled,lulled,ladles,
Toucan is the word.
Deline
Some six letter words with the fourth letter U are:abductaboundabruptabsurdaccuseacquitactualadjustalludeallureambushamountampuleannualaroundarousearoundarouseassumeassureastuteauburnaugustautumnbeautybelugabemuseblousecasualclausecloudycolumncrouchdebunkdeducedeductdefusedeludedelugedeluxedemuredeputydiluteeffuseendureengulfenoughensureeunuchexcuseexhumefecundfibulafigureflauntfuturegerundgrouchgroundgrouseimmuneimmureimpugnimpureimputeinduceinductinfuseinjureinsultinsurejaguarjejunejujubekabukikahunalegumeliquidliquorlocustluxurymanualmanurematureminuetminutemisusemodulemutualnaturenebulanoduleobtuseoccultoccupyonrushpenuryperuserebuffrebukereducerefugerefundrefuserefutereputeresultresumereturnritualrobustrotundsalutesecureseducesequelsequinsexualshouldshrunksleuthslouchspousesprucesprungstruckstrungsuburbsuturetenurethoughthrushthrusttraumatroughtroupetubuletumultuncurlundulyunfurlunjustunrulyunsungunsureupturnvacuumvicunavisualvolumeyogurt